Liquid crystal display device

ABSTRACT

A method of manufacturing, with high mass productivity, liquid crystal display devices having highly reliable thin film transistors with excellent electric characteristics is provided. In a liquid crystal display device having an inverted staggered thin film transistor, the inverted staggered thin film transistor is formed as follows: a gate insulating film is formed over a gate electrode; a microcrystalline semiconductor film which functions as a channel formation region is formed over the gate insulating film; a buffer layer is formed over the microcrystalline semiconductor film; a pair of source and drain regions are formed over the buffer layer; and a pair of source and drain electrodes are formed in contact with the source and drain regions so as to expose a part of the source and drain regions.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a liquid crystal display device using athin film transistor at least in a pixel portion.

2. Description of the Related Art

In recent years, techniques to form thin film transistors using, for achannel formation region, a semiconductor thin film (with a thickness ofapproximately several tens to several hundreds of nanometers) which isformed over a substrate having an insulating surface have attractedattention. Thin film transistors are widely used in electronic devicessuch as ICs and electro-optical devices, and their developmentespecially as switching elements for image display devices has beenaccelerated.

As a switching element of an image display device, a thin filmtransistor using an amorphous semiconductor film for a channel formationregion, a thin film transistor using a polycrystalline semiconductorfilm for a channel formation region, and the like are known. As a methodof forming a polycrystalline semiconductor film, there is known atechnique in which a pulsed excimer laser beam is shaped into a linearlaser beam by an optical system and an amorphous silicon film iscrystallized by being irradiated while being scanned with the linearlaser beam.

As a switching element of an image display device, a thin filmtransistor using a microcrystalline semiconductor film for a channelformation region is also used (see References 1 and 2).

-   [Reference 1] Japanese Published Patent Application No. H4-242724-   [Reference 2] Japanese Published Patent Application No. 2005-49832

SUMMARY OF THE INVENTION

A thin film transistor using a polycrystalline semiconductor film for achannel formation region has advantages in that its field-effectmobility is two or more orders of magnitude greater than that of a thinfilm transistor using an amorphous semiconductor film for a channelformation region and a pixel portion of a semiconductor display deviceand peripheral driver circuits thereof can be formed over the samesubstrate. However, the process of the thin film transistor using apolycrystalline semiconductor film for a channel formation region ismore complex than a thin film transistor using an amorphoussemiconductor film for a channel formation region, becausecrystallization of a semiconductor film provided over a glass substrateis added; accordingly, there are problems in that yield is decreased andcost is increased.

There is also a problem in that surfaces of crystal grains of amicrocrystalline semiconductor film are easily oxidized. Therefore, whencrystal grains in a channel formation region are oxidized, oxide filmsare formed on the surfaces of the crystal grains and the oxide filmsbecome obstacles to carrier transfer, which causes a problem in thatelectric characteristics of a thin film transistor are impaired.

In view of the above-mentioned problems, it is an object of the presentinvention to provide liquid crystal display devices having highlyreliable thin film transistors with good electric characteristics and amethod for manufacturing the liquid crystal display devices with highmass productivity.

In a liquid crystal display device having an inverted staggered thinfilm transistor, the inverted staggered thin film transistor is formedas follows: a gate insulating film is formed over a gate electrode; amicrocrystalline semiconductor film (also referred to as asemi-amorphous semiconductor film) which functions as a channelformation region is formed over the gate insulating film; a buffer layeris formed over the microcrystalline semiconductor film; a pair of sourceand drain regions are formed over the buffer layer; and a pair of sourceand drain electrodes are formed in contact with the source and drainregions so as to expose parts of the source and drain regions. Thus, thesource and drain regions include regions that are in contact with thesource and drain electrodes and regions that are not in contact with thesource and drain electrodes. Outside the source and drain electrodes,parts of the source and drain regions and a part of the buffer layer areexposed. The source and drain electrodes are not overlapped with endportions of the microcrystalline semiconductor film and the source anddrain regions. Outside end portions of the source and drain electrodes,end portions of the source and drain regions and the buffer layer areformed.

Due to misalignment of the end portions of the source and drainelectrodes with the end portions of the source and drain regions, anddue to formation of the end portions of the source and drain regionsoutside the end portions of the source and drain electrodes, the endportions of the source and drain electrodes are apart from each other;accordingly, leakage current and short circuit between the source anddrain electrodes can be prevented. In addition, an electric field can beprevented from being concentrated on the end portions of the source anddrain electrodes and the source and drain regions, and leakage currentbetween the gate electrode and the source and drain electrodes can beprevented.

The buffer layer has a concave portion in its part, and the side of theconcave portion is aligned with the end portions of the source and drainregions. Because the buffer layer has a concave in its part, the lengthbetween the source and drain regions is long, and thus the length of apath of carriers transfer is long, leakage current between the sourceand drain regions can be reduced.

Between the microcrystalline semiconductor film and the source and drainregions, the buffer layer is formed. The microcrystalline semiconductorfilm functions as a channel formation region. The buffer layer functionsas a high-resistance region as well as preventing the microcrystallinesemiconductor film from being oxidized. Because the buffer layer isformed using an amorphous semiconductor film having high resistancebetween the microcrystalline semiconductor film and the source and drainregions, field-effect mobility of a thin film transistor of the presentinvention is high, leakage current is low, and drain withstand voltageis high in the case of an off state (when a negative voltage is appliedto a gate electrode).

As the buffer layer, an amorphous semiconductor film can be used, andmoreover, it is preferable that the buffer layer be an amorphoussemiconductor film containing at least any one of nitrogen, hydrogen,and halogen. When an amorphous semiconductor film is made to contain anyone of nitrogen, hydrogen, and halogen, crystal grains contained in themicrocrystalline semiconductor film can be further prevented from beingoxidized.

The buffer layer can be formed by a plasma CVD method, a sputteringmethod, or the like. In addition, after an amorphous semiconductor filmis formed, the amorphous semiconductor film was subjected to nitrogenplasma, hydrogen plasma, or halogen plasma so that the amorphoussemiconductor film can be nitrided, hydrogenated or halogenated.

By provision of the buffer layer over the surface of themicrocrystalline semiconductor film, oxidation of crystal grainscontained in the microcrystalline semiconductor film can be reduced.Accordingly, the degree of degradation of electric characteristics of athin film transistor can be lowered.

Unlike a polycrystalline semiconductor film, a microcrystallinesemiconductor film can be directly formed over a substrate as amicrocrystalline semiconductor film. Specifically, a microcrystallinesemiconductor film can be formed using silicon hydride as a source gasand using a plasma CVD apparatus. The microcrystalline semiconductorfilm manufactured by the above method includes a microcrystallinesemiconductor film which contains crystal grains of 0.5 nm to 20 nm inan amorphous semiconductor. Thus, unlike in the case of using apolycrystalline semiconductor film, there is no need to conduct acrystallization process after formation of a semiconductor film. Thenumber of steps in manufacture of a thin film transistor can be reduced;yield of a liquid crystal display device can be increased; and cost canbe lowered. Plasma using a microwave with a frequency of 1 GHz or morehas high electron density, which facilitates dissociation of siliconhydride that is a source gas. Therefore, compared to a microwave plasmaCVD method with a frequency of several tens to several hundreds ofmegahertz, the microcrystalline semiconductor film can be formed moreeasily and deposition rate can be increased, by a plasma CVD methodusing a microwave with a frequency of 1 GHz or higher. Thus, the massproductivity of liquid crystal display devices can be increased.

In addition, thin film transistors (TFTs) are formed using amicrocrystalline semiconductor film, and a liquid crystal display deviceis manufactured using the thin film transistors in a pixel portion andalso in driver circuits. Because thin film transistors using amicrocrystalline semiconductor film each have a field-effect mobility of1 cm²/V·sec to 20 cm²/V·sec, which is 2 to 20 times greater than that ofa thin film transistor using an amorphous semiconductor film for itschannel forming region, some of or all of the driver circuits can beformed over the same substrate as the pixel portion to form asystem-on-panel display.

In addition, a liquid crystal display device includes an liquid crystalelement, Further, a liquid crystal display device includes a panel inwhich a liquid crystal is sealed, and a module in which an IC or thelike including a controller is mounted to the panel. Moreover, thepresent invention relates to an element substrate which is a mode beforecompletion of a liquid crystal element in a manufacturing process of theliquid crystal display device, and the element substrate includes ameans for supplying voltage to a liquid crystal element in each ofplural pixels. The element substrate may be specifically in a statewhere only a pixel electrode of a liquid crystal element is formed or ina state after a conductive film to be a pixel electrode is formed andbefore the conductive film is etched into a pixel electrode, and anymode is possible.

Note that the term “liquid crystal display device” in this specificationrefer to image display devices, the liquid crystal display devices andlight sources (including lighting devices). In addition, liquid crystaldisplay devices include all of the following modules: modules providedwith a connector, for example, a flexible printed circuit (FPC), a tapeautomated bonding (TAB) tape, or a tape carrier package (TCP); modulesprovided with a printed wiring board at the end of a TAB tape or a TCP;and modules where an integrated circuit (IC) is directly mounted on aliquid crystal element by a chip-on-glass (COG) method.

According to the present invention, it is possible to manufacture, withhigh mass productivity, liquid crystal display devices having highlyreliable thin film transistors with good electric characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 2A to 2C are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 3A and 3B are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 4A and 4B are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 5A to 5C are top views showing a method for manufacturing a liquidcrystal display device according to an aspect of the present invention;

FIGS. 6A to 6C are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 7A to 7C are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 8A and 8B are cross-sectional views showing a method formanufacturing a liquid crystal display device according to an aspect ofthe present invention;

FIGS. 9A to 9D are top views showing a method for manufacturing a liquidcrystal display device according to an aspect of the present invention;

FIG. 10 is a top view showing a microwave plasma CVD apparatus accordingto an aspect of the present invention;

FIGS. 11A to 11D are cross-sectional views showing multi-tone maskswhich can be applied to the present invention;

FIGS. 12A to 12C are perspective views showing liquid crystal displaypanels according to an aspect of the present invention;

FIGS. 13A to 13C are perspective views showing electronic devices eachusing a liquid crystal display device according to an aspect of thepresent invention;

FIG. 14 is a diagram showing an electronic device using a liquid crystaldisplay device according to an aspect of the present invention;

FIG. 15 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 16 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 17 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 18 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 19 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 20 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 21 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 22 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 23 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 24 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 25 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 26 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 27 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIG. 28 is a diagram showing a liquid crystal display device accordingto an aspect of the present invention;

FIGS. 29A and 29B are a top view and a cross-sectional view showing aliquid crystal display panel according to an aspect of the presentinvention, respectively;

FIG. 30 is a block diagram showing a structure of a liquid crystaldisplay device according to an aspect of the present invention;

FIG. 31 is an equivalent circuit diagram showing a structure of a drivercircuit of a liquid crystal display device according to an aspect of thepresent invention;

FIG. 32 is an equivalent circuit diagram showing a structure of a drivercircuit of a liquid crystal display device according to an aspect of thepresent invention;

FIG. 33 is a top view showing a layout of a driver circuit of a liquidcrystal display device according to an aspect of the present invention;

FIGS. 34A and 34B are diagrams showing results of measurement of amicrocrystalline semiconductor film by Raman spectroscopy;

FIG. 35 is a model diagram showing a device structure used for devicesimulation;

FIG. 36 is a graph showing current-voltage characteristics by devicesimulation; and

FIGS. 37A and 37B are diagrams each showing an electron concentrationdistribution of a thin film transistor obtained by device simulation.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be hereinafter describedwith reference to the drawings. However, the present invention can becarried out in many different modes, and it is easily understood bythose skilled in the art that the mode and detail of the presentinvention can be modified in various ways without departing from thespirit and scope thereof. Therefore, the present invention should not beinterpreted as being limited to the description in the followingembodiment modes.

Embodiment Mode 1

In this embodiment mode, manufacturing processes of thin filmtransistors used for a liquid crystal display device are described withreference to FIGS. 1A, 1B, FIGS. 2A to 2C, FIGS. 3A, 3B, FIGS. 4A, 4B,FIGS. 5A to 5C, FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A, 8B, FIGS. 9Ato 9D, FIG. 10, FIGS. 11A to 11D, and FIGS. 12A to 12C. FIGS. 1A, 1B,FIGS. 2A to 2C, FIGS. 3A, 3B, and FIGS. 4A, 4B and FIGS. 6A to 6C, FIGS.8A, 8B are cross-sectional views showing manufacturing processes of thinfilm transistors, and FIGS. 5A to 5C and FIGS. 9A to 9D are top viewseach showing a connection region of a thin film transistor and a pixelelectrode in a single pixel.

An n-channel thin film transistor having a microcrystallinesemiconductor film is more suitable for use in a driver circuit thanthat of a p-channel thin film transistor having a microcrystallinesemiconductor film, because the n-channel one has a higher field-effectmobility. It is desired that all thin film transistors formed over thesame substrate have the same polarity, in order to reduce the number ofsteps. Here, description is made using an n-channel thin filmtransistor.

As illustrated in FIG. 1A, a gate electrode 51 is formed over asubstrate 50. As the substrate 50, any of the following substrates canbe used: non-alkaline glass substrates made of barium borosilicateglass, aluminoborosilicate glass, aluminosilicate glass, and the like bya fusion method or a float method; ceramic substrates; plasticsubstrates having heat resistance enough to withstand a processtemperature of this manufacturing process; and the like. Alternatively,metal substrates of a stainless alloy and the like with the surfaceprovided with an insulating film may be employed. When the substrate 50is mother glass, the substrate may have any of the following sizes: thefirst generation (320 mm×400 mm), the second generation (400 mm×500 mm),the third generation (550 mm×650 mm), the fourth generation (680 mm×880mm, or 730 mm×920 mm), the fifth generation (1000 mm×1200 mm, or 1100mm×1250 mm), the sixth generation (1500 mm×1800 mm), the seventhgeneration (1900 mm×2200 mm), the eighth generation (2160 mm×2460 mm),the ninth generation (2400 mm×2800 mm, or 2450 mm×3050 mm), the tenthgeneration (2950 mm×3400 mm), and the like.

The gate electrode 51 is formed using a metal material such as titanium,molybdenum, chromium, tantalum, tungsten, or aluminum or an alloymaterial thereof. The gate electrode 51 can be formed in such a mannerthat a conductive film is formed over the substrate 50 by a sputteringmethod or a vacuum evaporation method; a mask is formed over theconductive film by a photolithography technique or an inkjet method; andthe conductive film is etched using the mask. Note that, as barriermetal which increases adhesion of the gate electrode 51 and preventsdiffusion to a base, a nitride film of the above-mentioned metalmaterial may be provided between the substrate 50 and the gate electrode51. Here, the gate electrode 51 is formed by etching of the conductivefilm formed over the substrate 50 with use of a resist mask formed usinga first photomask.

Note that, because an insulating film, a semiconductor film, a wiringand the like are to be formed over the gate electrode 51, it is desiredthat the gate electrode 51 be processed so as to have tapered endportions in order to prevent disconnection. In addition, although notshown, in this step, a wiring connected to the gate electrode can alsobe formed at the same time.

Next, over the gate electrode 51, gate insulating films 52 a and 52 b, amicrocrystalline semiconductor film 53, a buffer layer 54, asemiconductor film 55 to which an impurity element imparting oneconductivity type is added, and conductive films 65 a to 65 c are formedin this order. Then, a resist 80 is applied over the conductive film 65c. Note that it is preferable that at least the gate insulating films 52a and 52 b, the microcrystalline semiconductor film 53, and the bufferlayer 54 be formed successively. It is further preferable that the gateinsulating films 52 a and 52 b, the microcrystalline semiconductor film53, the buffer layer 54, and the semiconductor film 55 to which animpurity element imparting one conductivity type is added be formedsuccessively. By successive formation of at least the gate insulatingfilms 52 a and 52 b, the microcrystalline semiconductor film 53, and thebuffer layer 54 without any exposure to the atmosphere, each interfacebetween stacked layers can be formed without being contaminated by anatmospheric constituent or a contaminant impurity element floating inthe atmosphere. Thus, variations in characteristics of thin filmtransistors can be reduced.

The gate insulating films 52 a and 52 b can each be formed by a CVDmethod, a sputtering method, or the like using a silicon oxide film, asilicon nitride film, a silicon oxynitride film, or a silicon nitrideoxide film. Here, a mode is described in which a silicon oxide film or asilicon oxynitride film, and a silicon nitride film or a silicon nitrideoxide film are stacked in this order as the gate insulating films 52 aand 52 b. Note that the gate insulating film can be formed by stackingnot two layers but three layers of a silicon nitride film or a siliconnitride oxide film, a silicon oxide film or a silicon oxynitride film,and a silicon nitride film or a silicon nitride oxide film in this orderfrom the substrate side. Alternatively, the gate insulating film can beformed of a single layer of a silicon oxide film, a silicon nitridefilm, a silicon oxynitride film, or a silicon nitride oxide film.

Note that a silicon oxynitride film means a film that contains moreoxygen than nitrogen and, in the case where measurements are performedusing Rutherford backscattering spectrometry (RBS) and hydrogen forwardscattering (HFS), includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 50 at. % to 70 at. %, 0.5 at. % to 15 at. %,25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively. Further,a silicon nitride oxide film means a film that contains more nitrogenthan oxygen and, in the case where measurements are performed using RBSand HFS, includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 5 at. % to 30 at. %, 20 at. % to 55 at. %,25 at. % to 35 at. %, and 10 at. % to 30 at. %, respectively. Note thatpercentages of nitrogen, oxygen, silicon, and hydrogen fall within theranges given above, where the total number of atoms contained in thesilicon oxynitride film or the silicon nitride oxide film is defined as100 at. %.

The microcrystalline semiconductor film 53 is a film which contains asemiconductor having an intermediate structure between amorphous andcrystalline structures (including a single crystal and a polycrystal).This semiconductor is a semiconductor which has a third state that isstable in terms of free energy, and is a crystalline semiconductor whichhas short-range order and lattice distortion, and column-like orneedle-like crystals with its grains with a size of 0.5 nm to 20 nmgrown in the direction of a normal line with respect to the surface ofthe substrate. In addition, a microcrystalline semiconductor and anamorphous semiconductor are mixed. Microcrystalline silicon, which is atypical example of a microcrystalline semiconductor, has a Ramanspectrum which is shifted to a lower wave number side than 521 cm⁻¹ thatis a feature of single crystalline silicon. That is, the peak of a Ramanspectrum of microcrystalline silicon is within the range from 481 cm⁻¹to 521 cm⁻¹, which features of amorphous silicon and single crystallinesilicon respectively. In addition, microcrystalline silicon is made tocontain hydrogen or halogen of at least 1 at. % or more for terminationof dangling bonds. Moreover, microcrystalline silicon is made to containa rare gas element such as helium, argon, krypton, or neon to furtherenhance its lattice distortion, whereby stability is increased and afavorable microcrystalline semiconductor film can be obtained. Such amicrocrystalline semiconductor film is disclosed in, for example, U.S.Pat. No. 4,409,134.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens toseveral hundreds of megahertz or a microwave plasma CVD apparatus with afrequency of 1 GHz or more. The microcrystalline semiconductor film canbe typically formed using silicon hydride, such as SiH₄ or Si₂H₆, whichis diluted with hydrogen. With a dilution with one or plural kinds ofrare gas elements of helium, argon, krypton, and neon in addition tosilicon hydride and hydrogen, the microcrystalline semiconductor filmcan be formed. In that case, the flow rate ratio of hydrogen to siliconhydride is set to be 50:1 to 1000:1, preferably, 50:1 to 200:1, morepreferably, 100:1. Note that, in place of silicon hydride, SiH₂Cl₂,SiHCl₃, SiCl₄, SiF₄, or the like can be used.

A microcrystalline semiconductor film exhibits weak n-type conductivitywhen an impurity element for valence control is not intentionally added.Thus, threshold control of a microcrystalline semiconductor film whichfunctions as a channel formation region of a thin film transistor can bedone by addition of an impurity element which imparts p-typeconductivity at the same time as or after the film formation. A typicalexample of an impurity element which imparts p-type conductivity isboron, and an impurity gas such as B₂H₆ or BF₃ may be mixed into siliconhydride at a proportion of 1 ppm to 1000 ppm, preferably, 1 ppm to 100ppm. The concentration of boron may be set to be, for example, 1×10¹⁴atoms/cm³ to 6×10¹⁶ atoms/cm³.

In addition, the oxygen concentration of the microcrystallinesemiconductor film is preferably 5×10¹⁹ cm⁻³ or less, more preferably,1×10¹⁹ cm⁻³ or less and each of the nitrogen concentration and thecarbon concentration is preferably 3×10¹⁸ cm⁻³ or less. By decreases inconcentrations of oxygen, nitrogen, and carbon mixed into themicrocrystalline semiconductor film, the microcrystalline semiconductorfilm can be prevented from being changed into an n type.

The microcrystalline semiconductor film 53 is formed to a thickness ofgreater than 0 nm and less than or equal to 200 nm, preferably, from 1nm to 100 nm, more preferably, from 5 nm to 50 nm. The microcrystallinesemiconductor film 53 functions as a channel formation region of a thinfilm transistor to be formed later. When the thickness of themicrocrystalline semiconductor film 53 is within the range from 5 nm to50 nm, inclusive, the thin film transistor to be formed later is to be afully depleted type. In addition, because the deposition rate of themicrocrystalline semiconductor film 53 is low, i.e., a tenth to athousandth of the deposition rate of an amorphous semiconductor film, adecrease of thickness leads to an increase of throughput. Furthermore,because the microcrystalline semiconductor film contains microcrystals,it has a lower resistance than an amorphous semiconductor film.Therefore, a thin film transistor using the microcrystallinesemiconductor film for its channel formation region has current-voltagecharacteristics represented by a curve with a steep slope in a risingportion, has an excellent response as a switching element, and can beoperated at high speed. With the use of the microcrystallinesemiconductor film in a channel formation region of a thin filmtransistor, fluctuation of a threshold voltage of a thin film transistorcan be suppressed. Therefore, a liquid crystal display device with lessvariation of electrical characteristics can be manufactured.

The microcrystalline semiconductor film has a higher mobility than anamorphous semiconductor film. Thus, with use of a thin film transistor,a channel formation region of which is formed of the microcrystallinesemiconductor film, for switching of a liquid crystal element, the areaof the channel formation region, that is, the area of the thin filmtransistor can be decreased. Accordingly, the area occupied by the thinfilm transistor in a single pixel is decreased, and an aperture ratio ofthe pixel can be increased. As a result of this, a liquid crystaldisplay device with high resolution can be manufactured.

The buffer layer 54 can be formed by a plasma CVD method using siliconhydride such as SiH₄ or Si₂H₆. Alternatively, with a dilution of siliconhydride mentioned above with one or plural kinds of rare gas elementsselected from helium, argon, krypton, and neon, an amorphoussemiconductor film can be formed. With use of hydrogen at a flow ratewhich is 1 to 20 times, preferably, 1 to 10 times, more preferably, 1 to5 times higher than that of silicon hydride, a hydrogen-containingamorphous semiconductor film can be formed. With the use of siliconhydride mentioned above and nitrogen or ammonia, a nitrogen-containingamorphous semiconductor film can be formed. With use of silicon hydridementioned above and a gas containing fluorine, chlorine, bromine, oriodine (F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, HI, or the like), an amorphoussemiconductor film containing fluorine, chlorine, bromine, or iodine canbe formed. Note that, in place of silicon hydride, SiH₂Cl₂, SiHCl₃,SiCl₄, SiF₄, or the like can be used.

Alternatively, as the buffer layer 54, an amorphous semiconductor filmcan be formed by sputtering with hydrogen or a rare gas using anamorphous semiconductor as a target. In this case, by inclusion ofammonia, nitrogen, or N₂O in an atmosphere, a nitrogen-containingamorphous semiconductor film can be formed. Alternatively, by inclusionof a gas including fluorine, chlorine, bromine, or iodine (F₂, Cl₂, Br₂,I₂, HF, HCl, HBr, HI, or the like) in an atmosphere, an amorphoussemiconductor film containing fluorine, chlorine, bromine, or iodine canbe formed.

Still alternatively, the buffer layer 54 may be formed by formation ofan amorphous semiconductor film on the surface of the microcrystallinesemiconductor film 53 by a plasma CVD method or a sputtering method andthen by hydrogenation, nitridation, or halogenation of the surface ofthe amorphous semiconductor film through processing of the surface ofthe amorphous semiconductor film with hydrogen plasma, nitrogen plasma,or halogen plasma. Yet alternatively, the surface of the amorphoussemiconductor film may be processed with helium plasma, neon plasma,argon plasma, krypton plasma, or the like.

The buffer layer 54 is preferably formed using an amorphoussemiconductor film which does not contain crystal grains. Therefore,when the buffer layer 54 is formed by a high-frequency plasma CVD methodor a microwave plasma CVD method with a frequency of several tens toseveral hundreds of megahertz, formation conditions are preferablycontrolled such that an amorphous semiconductor film does not containcrystal grains.

The buffer layer 54 is partly etched in a later formation process of asource region and a drain region in some cases, and in that case, thebuffer layer 54 is preferably formed at such a thickness that a part ofthe buffer layer 54 is left after the etching. Typically, the thicknessof the buffer layer is preferably from 150 nm to 400 nm, inclusive. In aliquid crystal display device in which application voltage to a thinfilm transistor is high (e.g., approximately 15 V), by setting thethickness of the buffer layer 54 within the above range, the withstandvoltage is enhanced, and even when a high voltage is applied to a thinfilm transistor, the thin film transistor can be prevented fromdeteriorating.

Note that it is preferable that an impurity element imparting oneconductivity type such as phosphorus or boron be not added to the bufferlayer 54. In particular, it is preferable that boron contained in themicrocrystalline semiconductor film for threshold control or phosphoruscontained in the semiconductor film to which an impurity elementimparting one conductivity type is added be not mixed into the bufferlayer 54. As a result of this, by elimination of a region where leakagecurrent is generated due to a PN junction, leakage current can bedecreased. By formation of an amorphous semiconductor film, to which animpurity element imparting one conductivity type such as phosphorus orboron is not added, between the semiconductor film to which an impurityelement imparting one conductivity type is added and themicrocrystalline semiconductor film, the diffusion of the impuritycontained in each of the microcrystalline semiconductor film and sourceand drain regions can be prevented.

By formation of an amorphous semiconductor film, moreover, an amorphoussemiconductor film containing hydrogen, nitrogen, or halogen on thesurface of the microcrystalline semiconductor film 53, surfaces ofcrystal grains contained in the microcrystalline semiconductor film 53can be prevented from being oxidized naturally. In particular, in aregion where an amorphous semiconductor is in contact with microcrystalgrains, a crack is likely to be caused due to distortion of crystallattice. When this crack is exposed to oxygen, the crystal grains areoxidized, whereby silicon oxide is formed. However, by formation of thebuffer layer on the surface of the microcrystalline semiconductor film53, the microcrystal grains can be prevented from being oxidized.Furthermore, by formation of the buffer layer, the microcrystallinesemiconductor film can be prevented from being mixed with an etchingresidue which is generated in forming source and drain regions later.

The buffer layer 54 is formed using an amorphous semiconductor film oran amorphous semiconductor film containing hydrogen, nitrogen, orhalogen. An amorphous semiconductor film has a larger energy gap thanthe microcrystalline semiconductor film (the energy gap of the amorphoussemiconductor film is 1.1 eV to 1.5 eV and the energy gap of themicrocrystalline semiconductor film is 1.6 eV to 1.8 eV) and has ahigher resistance, and has a lower mobility, i.e., a fifth to a tenth ofthat of the microcrystalline semiconductor film. Therefore, in a thinfilm transistor to be formed later, the buffer layer formed betweensource and drain regions and the microcrystalline semiconductor filmfunctions as a high-resistant region and the microcrystallinesemiconductor film functions as a channel formation region. Accordingly,the off current of the thin film transistor can be reduced. When thethin film transistor is used as a switching element of a liquid crystaldisplay device, the contrast of the liquid crystal display device can beimproved.

As for the semiconductor film 55 to which an impurity element impartingone conductivity type is added, in the case where an n-channel thin filmtransistor is to be formed, phosphorus may be added as a typicalimpurity element, and an impurity gas such as PH₃ may be added tosilicon hydride. In the case where a p-channel thin film transistor isto be formed, boron may be added as a typical impurity element, and animpurity gas such as B₂H₆ may be added to silicon hydride. Thesemiconductor film 55 to which an impurity element imparting oneconductivity type is added can be formed of a microcrystallinesemiconductor or an amorphous semiconductor. Further, the semiconductorfilm 55 to which an impurity element imparting one conductivity type isadded may be a multilayer in which an amorphous semiconductor film towhich an impurity element imparting one conductivity type is added and amicrocrystalline semiconductor film to which an impurity elementimparting one conductivity type is added are stacked. An amorphoussemiconductor film to which an impurity element imparting oneconductivity type is added is formed on the buffer layer 54 side and amicrocrystalline semiconductor film to which an impurity elementimparting one conductivity type is added are stacked thereover, so thatresistance varies gradually, and thus carriers easily move and themobility can be enhanced. The semiconductor film 55 to which an impurityelement imparting one conductivity type is added is formed to athickness of from 2 nm to 50 nm, inclusive. By formation of thesemiconductor film to which an impurity element imparting oneconductivity type is added to a small thickness, throughput can beimproved.

Here, a plasma CVD apparatus, with which from the gate insulating films52 a and 52 b to the semiconductor film 55 to which an impurity elementimparting one conductivity type is added can be formed successively, isdescribed with reference to FIG. 10. FIG. 10 is a schematic diagramshowing an upper cross-sectional view of a plasma CVD apparatus, whichhas a structure where a loading chamber 1110, an unloading chamber 1115,and reaction chambers (1) 1111 to (4) 1114 are provided around a commonchamber 1120. Between the common chamber 1120 and the other chambers,gate valves 1122 to 1127 are provided so that processes performed in thechambers do not interface with each other. Substrates are loaded into acassette 1128 in the loading chamber 1110 and a cassette 1129 in theunloading chamber 1115 and carried to the reaction chambers (1) 1111 to(4) 1114 with a transport means 1121 of the common chamber 1120. In thisapparatus, a reaction chamber can be allocated for each of differentkinds of deposition films, and a plurality of different films can beformed successively without any exposure to the atmosphere.

In each of the reaction chambers (1) to (4), the gate insulating films52 a and 52 b, the microcrystalline semiconductor film 53, the bufferlayer 54, and the semiconductor film 55 to which an impurity elementimparting one conductivity type is added are stacked. In this case, aplurality of films of different kinds can be stacked successively bychange of source gases. In this case, after the gate insulating filmsare formed, silicon hydride such as silane is introduced into thereaction chamber, residual oxygen and silicon hydride are reacted witheach other, and the reactant is exhausted from the reaction chamber,whereby the concentration of residual oxygen in the reaction chamber canbe decreased. Accordingly, the concentration of oxygen to be containedin the microcrystalline semiconductor film can be decreased. Inaddition, crystal grains contained in the microcrystalline semiconductorfilm can be prevented from being oxidized.

Alternatively, the gate insulating films 52 a and 52 b, themicrocrystalline semiconductor film 53, and the buffer layer 54 areformed in each of the reaction chambers (1) and (3), and thesemiconductor film 55 to which an impurity element imparting oneconductivity type is added is formed in each of the reaction chambers(2) and (4). By formation of a film with an impurity element impartingone conductivity type alone, the impurity imparting one conductivitytype which remains in the chamber can be prevented from being mixed intoanother film.

In this manner, with a microwave plasma CVD apparatus where a pluralityof chambers is connected, the gate insulating films 52 a and 52 b, themicrocrystalline semiconductor film 53, the buffer layer 54, and thesemiconductor film 55 to which an impurity element imparting oneconductivity type is added can be formed at the same time. Thus, massproductivity can be improved. In addition, even when maintenance orcleaning is performed in one of reaction chambers, film formationprocesses can be performed in the other reaction chambers, whereby cycletime for film formation can be shortened. Furthermore, each interfacebetween stacked layers can be formed without being contaminated by anatmospheric constituent or a contaminant impurity element floating inthe atmosphere. Thus, variations in characteristics of thin filmtransistors can be reduced.

Alternatively, it is possible that the gate insulating films 52 a and 52b are formed in the reaction chamber (1), the microcrystallinesemiconductor film 53 and the buffer layer 54 are formed in the reactionchamber (2), and the semiconductor film 55 to which an impurity elementimparting one conductivity type is added is formed in the reactionchamber (3). Still alternatively, in the case where the gate insulatingfilm 52 a is to be formed of a silicon oxide film or a siliconoxynitride film and the gate insulating film 52 b is to be formed of asilicon nitride film or a silicon nitride oxide film, five reactionchambers may be provided. A silicon oxide film or a silicon oxynitridefilm may be formed as the gate insulating film 52 a in a reactionchamber (1); a silicon nitride film or a silicon nitride oxide film maybe formed as the gate insulating film 52 b in a reaction chamber (2); amicrocrystalline semiconductor film may be formed in a reaction chamber(3); a buffer layer may be formed in a reaction chamber (4); and asemiconductor film to which an impurity element imparting oneconductivity type is added may be formed in a reaction chamber (5). Thedeposition rate of a microcrystalline semiconductor film is low; thus,microcrystalline semiconductor films may be formed in a plurality ofreaction chambers. For example, the gate insulating films 52 a and 52 bmay be formed in a reaction chamber (1); the microcrystallinesemiconductor films 53 may be formed in reaction chambers (2) and (3);the buffer layer 54 may be formed in a reaction chamber (4); and thesemiconductor film 55 to which an impurity element imparting oneconductivity type is added may be formed in a reaction chamber (5). Inthis manner, by formation of the microcrystalline semiconductor films 53in a plurality of reaction chambers at the same time, throughput can beimproved. In this case, it is preferable that the inner wall of eachreaction chamber be coated with a film of the same kind as a film to beformed therein.

With use of a microwave plasma CVD apparatus having such a structure asdescribed above, films of similar kinds or a film of one kind can beformed in each reaction chamber and can be formed successively withoutany exposure to the atmosphere. Therefore, each interface betweenstacked layers can be formed without being contaminated by a residue ofthe previously formed film or an impurity element floating in theatmosphere.

Note that the microwave plasma CVD apparatus shown in FIG. 10 isprovided with the loading chamber and the unloading chamber separately,which may be a single loading/unloading chamber. In addition, the plasmaCVD apparatus may be provided with a spare chamber. By preheating of asubstrate in the spare chamber, heating time needed before filmformation in each reaction chamber can be shortened; thus, throughputcan be improved.

Film formation processes are described below. In each film formationprocess, a gas to be supplied from a gas supply portion may be selecteddepending on the purpose.

Here, a method is given as an example, in which a silicon oxynitridefilm is formed as the gate insulating film 52 a and a silicon nitrideoxide film is formed as the gate insulating film 52 b.

First, inside of a processing container in a reaction chamber of themicrowave plasma CVD apparatus is cleaned with fluorine radicals. Notethat the inside of the reaction chamber can be cleaned by introductionof fluorine radicals into the reaction chamber, which are generated byintroduction of carbon fluoride, nitrogen fluoride, or fluorine into aplasma generator provided outside of the reaction chamber and bydissociation thereof.

After cleaning with fluorine radicals, a large amount of hydrogen isintroduced into the reaction chamber, whereby residual fluorine insidethe reaction chamber can be reacted with hydrogen, so that theconcentration of residual fluorine can be decreased. Accordingly, theamount of fluorine to be mixed into a protective film that is to bedeposited later on the inner wall of the reaction chamber can bedecreased, and the thickness of the protective film can be decreased.

Next, on the surface of the inner wall of the processing container inthe reaction chamber, or the like, an oxynitride film is deposited asthe protection film. Here, the pressure in the processing container isset to be 1 Pa to 200 Pa, preferably, 1 Pa to 100 Pa, and one or morekinds of rare gases such as helium, argon, xenon, and krypton isintroduced as a plasma ignition gas. Furthermore, hydrogen and any onekind of rare gases are introduced. In particular, it is preferable thathelium, moreover, helium and hydrogen be used as a plasma ignition gas.

Although helium has a high ionization energy of 24.5 eV, it has ametastable state at about 20 eV. Therefore, helium can be ionized atabout 4 eV during discharge. Thus, discharge starting voltage is low anddischarge can be maintained easily. Accordingly, uniform plasma can bemaintained, and power saving can be achieved.

Alternatively, as the plasma ignition gas, an oxygen gas and one or morekinds of rare gases such as helium, argon, xenon, and krypton may beintroduced. By introduction of an oxygen gas together with a rare gasinto the processing container, plasma ignition can be facilitated.

Next, a power supply apparatus is turned on and the output of the powersupply apparatus is set to be 500 W to 6000 W, preferably, 4000 W to6000 W to generate plasma. Then, a source gas is introduced into theprocessing container through a gas pipe. Specifically, by introductionof dinitrogen monoxide, a rare gas, and silane as a source gas, asilicon oxynitride film is formed as the protective film on the innerwall of the processing container. The flow rate of silicon hydride atthis time is set to be 50 sccm to 300 sccm and that of dinitrogenmonoxide is set to be 500 sccm to 6000 sccm, and the thickness of theprotective film is set to be 500 nm to 2000 nm.

Then, the supply of the source gas is stopped, the pressure in theprocessing container is decreased, and the power supply apparatus isturned off. After that, a substrate is introduced onto the supportingbase in the processing container.

Next, through a similar process to that of the protective film, asilicon oxynitride film is deposited over the substrate as the gateinsulating film 52 a.

After the silicon oxynitride film is deposited to a predeterminedthickness, the supply of the source gas is stopped, the pressure in theprocessing container is decreased, and the power supply apparatus isturned off.

Next, the pressure in the processing container is set to be 1 Pa to 200Pa, preferably, 1 Pa to 100 Pa, and one or more kinds of rare gases suchas helium, argon, xenon, and krypton which is a plasma ignition gas andsilane, dinitrogen monoxide, and ammonia which are a source gas areintroduced. Note that, as the source gas, nitrogen may be introducedinstead of ammonia. Then, the power supply apparatus is turned on, andthe output of the power supply apparatus is set to be 500 W to 6000 W,preferably, 4000 W to 6000 W to generate plasma. Next, the source gas isintroduced into the processing container through the gas pipe, and asilicon nitride oxide film is formed as the gate insulating film overthe silicon oxynitride film over the substrate 1130. Then, the supply ofthe source gas is stopped, the pressure in the processing container isdecreased, the power supply apparatus is turned off, and the filmformation process is completed.

Through the above-described process, the silicon oxynitride film isformed as the protective film on the inner wall of the reaction chamber,and the silicon oxynitride film and the silicon nitride oxide film aresuccessively formed over the substrate, whereby mixture of an impuritysuch as silicon oxide into the silicon nitride oxide film on the upperlayer side can be suppressed. By formation of the above-mentioned filmsby a microwave plasma CVD method using a power supply apparatus whichcan generate a microwave as a power supply apparatus, plasma density canbe made to be high and the films can be formed to have high withstandvoltage. With use of the films as a gate insulating film, variations inthreshold voltages of transistors can be suppressed. In addition, BTcharacteristics can be improved. Moreover, resistance to staticelectricity is increased, and a transistor which is not easily destroyedeven when high voltage is applied can be manufactured. Furthermore, atransistor which is less destroyed over time can be manufactured. Inaddition, a transistor with less hot carrier damage can be manufactured.

In the case where a gate insulating film is a single layer of a siliconoxynitride film which is formed using the microwave plasma CVDapparatus, the above-described formation method of the protective filmand the formation method of the silicon oxynitride film are used. Inparticular, when the flow rate ratio of dinitrogen monoxide to silane isset to be 50:1 to 300:1, preferably, 50:1 to 250:1, the siliconoxynitride film can be formed to have high withstand voltage.

Next, a film formation process is described, in which a microcrystallinesemiconductor film and an amorphous semiconductor film as a buffer layerare successively formed by a plasma CVD method. First, in a similarmanner to the gate insulating films, the inside of the reaction chamberis cleaned. Next, a silicon film is deposited as a protective filminside the processing container. Here, the pressure in the processingcontainer is set to be 1 Pa to 200 Pa, preferably, 1 Pa to 100 Pa, andone or more kinds of rare gases such as helium, argon, xenon, andkrypton is introduced as a plasma ignition gas. Note that hydrogen maybe introduced together with the rare gas.

Then, the power supply apparatus is turned on, and the output of thepower supply apparatus is set to be 500 W to 6000 W, preferably, 4000 Wto 6000 W to generate plasma. Next, a source gas is introduced into theprocessing container through the gas pipe. Specifically, by introductionof a silicon hydride gas and a hydrogen gas as a source gas, amicrocrystalline silicon film is formed as a protective film on theinner wall of the processing container. Alternatively, amicrocrystalline semiconductor film can be formed from a dilution of asilicon hydride gas and a hydrogen gas with one or more kinds of raregas elements selected from helium, argon, krypton, and neon. The flowrate ratio of hydrogen to silicon hydride at this time is set to be 5:1to 1000:1, preferably, 50:1 to 200:1, more preferably, 100:1 to 150:1.In addition, the thickness of the protective film at this time is set tobe 500 nm to 2000 nm. Note that, before the power supply apparatus isturned on, a silicon hydride gas and a hydrogen gas in addition to theabove-mentioned rare gas may be introduced into the processingcontainer.

Alternatively, an amorphous semiconductor film can be formed as theprotective film from a dilution of a silicon hydride gas and a hydrogengas with one or more kinds of rare gas elements selected from helium,argon, krypton, and neon.

Then, the supply of the source gas is stopped, the pressure in theprocessing container is decreased, and the power supply apparatus isturned off. After that, the substrate is introduced onto the supportbase in the processing container.

Next, the surface of the gate insulating film 52 b formed over thesubstrate may be processed with hydrogen plasma. By processing withhydrogen plasma before the microcrystalline semiconductor film isformed, lattice distortion at the interface between the gate insulatingfilm and the microcrystalline semiconductor film can be decreased, andinterface characteristics of the gate insulating film and themicrocrystalline semiconductor film can be improved. Accordingly,electric characteristics of a thin film transistor to be formed latercan be improved.

In the hydrogen plasma processing, the amorphous semiconductor film orthe microcrystalline semiconductor film which is formed as theprotective film inside the processing container is also processed withhydrogen plasma, whereby the protective film is etched and a slightamount of semiconductor is deposited on the surface of the gateinsulating film 52 b. The semiconductor serves as a nucleus of crystalgrowth, and with the nucleus, a microcrystalline semiconductor film isdeposited. As a result, lattice distortion at the interface between thegate insulating film and the microcrystalline semiconductor film can bedecreased, and interface characteristics of the gate insulating film andthe microcrystalline semiconductor film can be improved. Accordingly,electric characteristics of a thin film transistor to be formed latercan be improved.

Next, in a similar process to the protective film, a microcrystallinesilicon film is deposited over the substrate. The thickness of themicrocrystalline silicon film is set to be greater than 0 nm and lessthan or equal to 50 nm, preferably, greater than 0 nm and less than orequal to 20 nm.

After the microcrystalline silicon film is deposited to a predeterminedthickness, the supply of the source gas is stopped, the pressure in theprocessing container is decreased, the power supply apparatus is turnedoff, and the film formation process for the microcrystallinesemiconductor film is completed.

Next, the pressure in the processing container is decreased, and theflow rate of the source gas is adjusted. Specifically, the flow rate ofa hydrogen gas is significantly decreased compared to that under filmformation conditions for the microcrystalline semiconductor film.Typically, a hydrogen gas is introduced at a flow rate which is 1 to 20times, preferably, 1 to 10 times, more preferably, 1 to 5 times higherthan the flow rate of silicon hydride. Alternatively, a silicon hydridegas is introduced without any hydrogen gas being introduced into theprocessing container. By such a decrease in flow rate of hydrogen tosilicon hydride, the deposition rate of an amorphous semiconductor filmas the buffer layer can be increased. Alternatively, a silicon hydridegas is diluted with one or more kinds of rare gas elements selected fromhelium, argon, krypton, and neon. Next, the power supply apparatus isturned on and the output of the power supply apparatus is set to be 500W to 6000 W, preferably, 4000 W to 6000 W to generate plasma 200,whereby an amorphous semiconductor film can be formed. Because thedeposition rate of an amorphous semiconductor film is higher than thatof a microcrystalline semiconductor film, the pressure in the processingcontainer can be set to be low. The thickness of the amorphoussemiconductor film at this time is set to be 200 nm to 400 nm.

After the amorphous semiconductor film is deposited to a predeterminedthickness, the supply of the source gas is stopped, the pressure in theprocessing container is decreased, the power supply apparatus is turnedoff, and the film formation process for the amorphous semiconductor filmis completed.

Note that the microcrystalline semiconductor film 53 and the amorphoussemiconductor film which is the buffer layer 54 may be formed whileplasma is ignited. Specifically, while the flow rate ratio of hydrogento silicon hydride which are the source gas used to form themicrocrystalline semiconductor film 53 is gradually decreased, themicrocrystalline semiconductor film 53 and the amorphous semiconductorfilm which is the buffer layer 54 are stacked. By such a method, animpurity is not deposited at the interface between the microcrystallinesemiconductor film 53 and the buffer layer 54; interface with lessdistortion can be formed; and electric characteristics of a thin filmtransistor to be formed later can be improved.

In the case where the microcrystalline semiconductor film 53 is formed,a microwave plasma CVD apparatus with a frequency of 1 GHz or more ispreferably used. Microwave plasma has high electron density, and a largenumber of radicals are formed from a source gas and supplied to thesubstrate 1130. Thus, reaction of radicals at the surface of thesubstrate is promoted, and the deposition rate of microcrystallinesilicon can be increased. By a plasma CVD method with a high frequencyof 1 MHz to 20 MHz, typically 13.56 MHz, or a high frequency in VHFband, from 20 MHz to about 120 MHz, typically, 27.12 MHz or 60 MHz, themicrocrystalline semiconductor film can be formed.

Note that, in each of the manufacturing processes of the gate insulatingfilm and the semiconductor film, in the case where a protective film of500 nm to 2000 nm is formed on the inner wall of the reaction chamber,the cleaning process and the formation process of the protective filmcan be omitted.

The conductive films 65 a to 65 c are formed over the semiconductor film55 to which an impurity element imparting one conductivity type isadded, using a single layer or a stacked layer of aluminum, copper or analuminum alloy to which an element to improve heat resistance or anelement to prevent a hillock, e. g., silicon, titanium, neodymium,scandium, or molybdenum, is added. Alternatively, the conductive filmmay have a stacked-layer structure where a film on the side in contactwith the semiconductor film to which an impurity element imparting oneconductivity type is added is formed of titanium, tantalum, molybdenum,tungsten, or nitride of any of these elements and an aluminum film or analuminum alloy film is formed thereover. Still alternatively, theconductive film may have a stacked-layer structure where an aluminumfilm or an aluminum alloy film is sandwiched between upper and lowerfilms of titanium, tantalum, molybdenum, tungsten, or nitride of any ofthese elements. Here, as the conductive film, a conductive film with athree-layer structure where conductive films 65 a to 65 c are stacked isdescribed. A stacked-layer conductive film where molybdenum films areused as the conductive films 65 a and 65 c and an aluminum film is usedas the conductive film 65 b or a stacked-layer conductive film wheretitanium films are used as the conductive films 65 a and 65 c and analuminum film is used as the conductive film 65 b can be given. Theconductive films 65 a to 65 c are formed by a sputtering method or avacuum evaporation method.

A resist 80 can be a positive type resist or a negative type resist. Inthis case, a positive type resist is used.

Next, the resist 80 is irradiated with light using a multi-tonephotomask 59 as a second photomask to expose the resist 80 to light.

Now light exposure using the multi-tone photomask 59 is described withreference to FIGS. 11A to 11D.

A multi-tone photomask can achieve three levels of light exposure, anexposed portion, a half-exposed portion, and an unexposed portion;one-time exposure and development process allows a resist mask withregions of plural thicknesses (typically, two kinds of thicknesses) tobe formed. The use of a multi-tone photomask allows the number ofphotomasks to be reduced.

Typical examples of a multi-tone photomask include a gray-tone mask 59 aas illustrated in FIG. 11A, and a half-tone mask 59 b as illustrated inFIG. 11C.

As illustrated in FIG. 11A, the gray-tone mask 59 a includes a substrate163 having a light-transmitting property, and a light-blocking portion164 and a diffraction grating 165 that are formed thereon. The lighttransmittance of the light-blocking portion 164 is 0%. The diffractiongrating 165 has a light-transmitting portion in a slit form, a dot form,a mesh form, or the like with intervals which are less than or equal tothe resolution limit for light used for the exposure; thus, the lighttransmittance can be controlled. The diffraction grating 165 can haveregularly-arranged slits, dots, or meshes form, or irregularly-arrangedslits, dots, or meshes.

For the substrate 163 having a light-transmitting property, a substratehaving a light-transmitting property, such as a quartz substrate, can beused. The light-blocking portion 164 and the diffraction grating 165 canbe formed using a light-blocking material such as chromium or chromiumoxide, which absorbs light.

When the gray-tone mask 59 a is irradiated with light for exposure, thelight transmittance 166 of the light-blocking portion 164 is 0% and thatof a region where neither the light-blocking portion 164 nor thediffraction grating 165 are provided is 100%, as illustrated in FIG.11B. The light transmittance of the diffraction grating 165 can becontrolled in a range of 10% to 70%. The light transmittance of thediffraction grating 165 can be controlled by adjusting an interval or apitch between slits, dots, or meshes of the diffraction grating 165.

As illustrated in FIG. 11C, the half-tone mask 59 b includes thesubstrate 163 having a light-transmitting property, and asemi-transmissive portion 167 and a light-blocking portion 168 that areformed thereon. The semi-transmissive portion 167 can be formed usingMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 168 can be formed using a light-blocking material such aschromium or chromium oxide, which absorbs light.

When the half-tone mask 59 b is irradiated with light for exposure, thelight transmittance 169 of the light-blocking portion 168 is 0% and thatof a region where neither the light-blocking portion 168 nor thesemi-transmissive portion 167 is provided is 100%, as illustrated inFIG. 11D. The light transmittance of the semi-transmissive portion 167can be controlled in a range of 10% to 70%. The light transmittance ofthe semi-transmissive portion 167 can be controlled depending on amaterial of the semi-transmissive portion 167.

After the light exposure using the multi-tone photomask is done,development is carried out, whereby the resist mask 81 having regionswith different thicknesses can be formed, as illustrated in FIG. 1B.

Next, with the resist mask 81, the microcrystalline semiconductor film53, the buffer layer 54, the semiconductor film 55 to which an impurityelement imparting one conductivity type is added, and the conductivefilms 65 a to 65 c are etched for separation. As a result, amicrocrystalline semiconductor film 61, a buffer layer 62, asemiconductor film 63 to which an impurity element imparting oneconductivity type is added, and conductive films 85 a to 85 c can beformed, as illustrated in FIG. 2A. Note that FIG. 2A corresponds to across-sectional view taken along the line A-B in FIG. 5A (excluding aresist mask 86).

The side faces in the end portions of the microcrystalline semiconductorfilm 61 and the buffer layer 62 are inclined, so that leakage currentcan be prevented from flowing between the source and drain regionsformed over the buffer layer 62 and the microcrystalline semiconductorfilm 61. In addition, leakage current between the source and drainelectrodes and the microcrystalline semiconductor film 61 can also beprevented. The inclination angle of the side faces in the end portionsof the microcrystalline semiconductor film 61 and the buffer layer 62 isfrom 30° to 90°, preferably from 45° to 80°. By adopting such an angle,disconnection of the source electrode or the drain electrode due to thestep can be prevented.

Next, ashing is conducted on the resist mask 81. As a result, the areaand the thickness of the resist are reduced. At this time, the resist ina region with a small thickness (a region overlapping with part of thegate electrode 51) is removed to form separated resist masks 86, asillustrated in FIG. 6B.

Next, using the resist mask 86, the semiconductor film 63 to which animpurity element imparting one conductivity type is added and theconductive films 85 a to 85 c are etched for separation. At this time,dry etching is conducted to separate the conductive films 85 a to 85 c.As a result, a pair of conductive films 89 a to 89 c and a pair ofsource and drain regions 89 can be formed, as illustrated in FIG. 2B. Inthis etching process, a part of the buffer layer 62 is also etched. Thebuffer layer which is partly etched is referred to as a buffer layer 88.The source and drain regions and the concave portion of the buffer layercan be formed in the same process. The buffer layer 88 has end portionsprotruding outside conductive films 85 a to 85 c because the bufferlayer 88 is partly etched using the resist mask 86 with a reduced area.

Next, as illustrated in FIG. 2C, the conductive films 89 a to 89 c arepartly etched to form source and drain electrodes 92 a to 92 c. Here, bywet etching of the conductive films 89 a to 89 c with use of the resistmask 86, the end portions of the conductive films 89 a to 89 c areselectively etched. As a result, the source and drain electrodes 92 a to92 c having smaller areas than the resist mask 86 and the conductivefilms 89 a to 89 c can be formed. The end portions of the source anddrain electrodes 92 a to 92 c are not aligned with the end portions ofthe source and drain regions 89, and the end portions of the source anddrain regions 89 are formed outside the end portions of the source anddrain electrodes 92 a to 92 c. After that, the resist mask 86 isremoved.

Note that FIG. 2C corresponds to a cross-sectional view taken along theline A-B of FIG. 5B. As illustrated in FIG. 5B, it can be seen that theend portions of the source and drain regions 89 are located outside ofthe end portions of the source and drain electrodes 92 c. In addition,the end portions of the buffer layer 88 are located outside the endportions of the source and drain electrodes 92 c and the source anddrain regions 89. Further, one of the source and drain electrodes has ashape in which the one electrode partially surrounds the other of thesource and drain regions (specifically, a U shape or a C shape). Thus,an area in which carriers can move can be increased, and thus the amountof current can be increased and an area for a thin film transistor canbe reduced. In the inner portion of the gate electrode, themicrocrystalline semiconductor film 87 and the source and drainelectrodes 92 c are overlapped, and thus influence by unevenness at theend portions of the gate electrode is small and reduction of coverageand generation of leakage current can be suppressed. Note that one ofthe source and drain electrodes also functions as a source or drainwiring.

With such a shape as illustrated in FIG. 2C in which the end portions ofthe source and drain electrodes 92 a to 92 c are not aligned with theend portions of the source and drain regions 89, the end portions of thesource and drain electrodes 92 a to 92 c are apart from each other;therefore, leakage current and short circuit between the source anddrain electrodes can be prevented. Accordingly, a thin film transistorwith high reliability and high withstand voltage can be manufactured.

Through the above-described process, a channel-etch thin film transistor83 can be formed. In addition, the thin film transistor can be formedusing two photomasks.

In the thin film transistor described in this embodiment mode, the gateinsulating film, the microcrystalline semiconductor film, the bufferlayer, the source and drain regions, and the source and drain electrodesare stacked over the gate electrode, and the buffer layer covers thesurface of the microcrystalline semiconductor film which functions as achannel formation region. In addition, a concave portion (a groove) isformed in a part of the buffer layer, and regions other than the concaveportion are covered with the source and drain regions. That is, due tothe concave formed in the buffer layer, the length of a path forcarriers the source and drain regions is long; thus, leakage currentbetween the source and drain regions can be reduced. In addition,because the concave is formed by etching of a part of the buffer layer,an etching residue which is generated in the formation step of thesource and drain regions can be removed. Accordingly, leakage current(parasitic channel) can be prevented from being generated between thesource and drain regions through the residue.

The buffer layer is formed between the microcrystalline semiconductorfilm which functions as a channel formation region and the source anddrain regions. In addition, the buffer layer covers the surface of themicrocrystalline semiconductor film. Because the buffer layer, which isformed to have high resistance, is extended to regions between themicrocrystalline semiconductor film and the source and drain regions,leakage current generated in the case where the thin film transistor isturned off (i.e., the case where a negative voltage is applied to thegate electrode) can be reduced, and deterioration due to application ofhigh voltage can be suppressed. Moreover, because the amorphoussemiconductor film, the surface of which is subjected to termination byhydrogen, is formed as the buffer layer on the surface of themicrocrystalline semiconductor film, the microcrystalline semiconductorfilm can be prevented from being oxidized, and an etching residue whichis generated in the formation step of the source and drain regions canbe prevented from being mixed into the microcrystalline semiconductorfilm. Accordingly, the thin film transistor has high electriccharacteristics and excellent drain withstand voltage.

Because the end portions of the source and drain electrodes are apartfrom each other due to the shape where the end portions of the sourceand drain electrodes are not aligned with the end portions of the sourceand drain regions, leakage current and short circuit between the sourceand drain electrodes can be prevented.

Next, as illustrated in FIG. 3A, an insulating film 76 is formed overthe source and drain electrodes 92 a to 92 c, the source and drainregions 89, the buffer layer 88, the microcrystalline semiconductor film87, and the gate insulating film 52 b. The insulating film 76 can beformed in a similar manner to the gate insulating films 52 a and 52 b.Note that the insulating film 76 is provided to prevent entry of acontaminant impurity such as an organic substance, a metal substance, ormoisture floating in the atmosphere and is preferably a dense film. Byuse of a silicon nitride film as the insulating film 76, the oxygenconcentration in the buffer layer 88 can be made to be 5×10¹⁹ atoms/cm³or less, preferably, 1×10¹⁹ atoms/cm³ or less.

Next, a contact hole is formed in the insulating film 76, and a pixelelectrode 77 is formed in the contact hole to be in contact with thesource or drain electrode 92 c. Note that FIG. 3B corresponds to across-sectional view taken along a line A-B of FIG. 5C.

The pixel electrode 77 can be formed using a light-transmittingconductive material such as indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, or indium tinoxide to which silicon oxide is added.

Alternatively, the pixel electrode 77 can be formed using a conductivecomposition containing a conductive high-molecular compound (alsoreferred to as a conductive polymer). It is preferable that the pixelelectrode formed using the conductive composition have a sheetresistance of 10000 Ω/square or less and a light transmittance of 70% ormore at a wavelength of 550 nm. In addition, it is preferable that theresistivity of the conductive high-molecular compound contained in theconductive composition be 0.1 Ω·cm or less.

As the conductive high-molecular compound, a so-called π electronconjugated conductive high-molecular compound can be used. Examplesinclude polyaniline and its derivatives, polypyrrole and itsderivatives, polythiophene and its derivatives, copolymers of two ormore kinds of them, and the like.

Through the above process, an element substrate which can be used for aliquid crystal display device can be formed.

As illustrated in FIG. 2A, the microcrystalline semiconductor film 61,the buffer layer 62, the semiconductor film 63 to which an impurityelement imparting one conductivity type is added, and the conductivefilms 85 a to 85 c are formed, and then as illustrated in FIG. 4A, theconductive films 85 a to 85 c are etched using the resist mask 86. Inthis case, the conductive films 85 a to 85 c are etched isotropicallyusing the resist mask 86, so that the exposed portions and vicinityportions thereof of the conductive film 85 a to 85 c are selectivelyetched. As a result, the source and drain electrodes 92 a to 92 c havingsmaller regions than the resist mask 86 can be formed.

Next, as illustrated in FIG. 4B, the semiconductor film 63 to which animpurity element imparting one conductivity type is added is etchedusing the resist mask 86. In this case, the semiconductor film 63 towhich an impurity element imparting one conductivity type is added isanisotropically etched by dry etching, so that the source and drainregions 89 having almost the same area as the resist mask 86 can beformed.

With such a shape in which the end portions of the source and drainelectrodes 92 a to 92 c and the end portions of the source and drainregions 89 are not aligned, the end portions of the source and drainelectrodes 92 a to 92 c are apart from each other; therefore, leakagecurrent and short circuit between the source and drain electrodes can beprevented. Accordingly, a thin film transistor with high reliability andhigh withstand voltage can be manufactured.

As illustrated in FIGS. 1A and 1B, FIGS. 2A to 2C, FIGS. 3A and 3B, andFIGS. 4A and 4B, the conductive film is etched by wet etching, and thesemiconductor film to which an impurity element imparting oneconductivity type is added is etched by dry etching, so that a structurein which the end portions of the source and drain electrodes and the endportions of the source and drain regions are not aligned and aredifferent be obtained with a small number of photomasks.

Next, a method for manufacturing a thin film transistor, which isdifferent from that in the above mode, is described with reference toFIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A and 8B, and FIGS. 9A to 9D.Here, a structure is described, in which a source or drain electrode isseparated from a source or drain wiring.

As illustrated in FIG. 6A, a gate electrode 51 is formed over asubstrate 50. Next, over the gate electrode 51, gate insulating films 52a and 52 b, a microcrystalline semiconductor film 53, a buffer layer 54,a semiconductor film 55 to which an impurity element imparting oneconductivity type is added, and a conductive film 65 a are sequentiallyformed. Then, over the conductive film 65 a, resist is applied, and aresist mask 81 having regions with different thicknesses is formed usinga multi-tone mask illustrated in FIG. 1A.

Next, the microcrystalline semiconductor film 53, the buffer layer 54,the semiconductor film 55 to which an impurity element imparting oneconductivity type is added, and the conductive film 65 a is etched usingthe resist mask 81 for separation. As a result, as illustrated in FIG.6B, a microcrystalline semiconductor film 61, a buffer layer 62, asemiconductor film 63 to which an impurity element imparting oneconductivity type is added, and a conductive film 85 a can be formed.Note that FIG. 6B corresponds to a cross-sectional view taken along theline A-B of FIG. 9A (excluding the resist mask 86).

Next, asking is conducted on the resist mask 81 to form separated resistmasks 86. Then, the semiconductor film 63 to which an impurity elementimparting one conductivity type is added and the conductive film 85 aare etched the resist mask 86 for separation. As a result, asillustrated in FIG. 6C, a pair of conductive films 89 a and a pair ofsource and drain regions 89 can be formed. Note that, in this etchingstep, a part of the buffer layer 62 is also etched. The partly etchedbuffer layer is referred to as a buffer layer 88. Here, because a partof the buffer layer 88 is etched using the resist mask 86 with a reducedarea, the buffer layer 88 has a shape where it extends outside theconductive film 85 a. As shown in this embodiment mode, the side face ofthe buffer layer is stepwise, and thus the coverage with an insulatingfilm to be formed later is increased. Therefore, leakage current betweena pixel electrode formed over the insulating film and a thin filmtransistor can be reduced.

Next, ashing is conducted on the resist mask 86. As a result, asillustrated in FIG. 7A, the area of the resist mask is reduced, and thethickness thereof is reduced. By using the ashed resist mask 91, theconductive film 89 a is partly etched, and as illustrated in FIG. 7B,the source and drain electrodes 92 a are formed. The end portions of thesource and drain electrodes 92 a are not aligned with the end portionsof the source and drain regions 89. In this case, an exposed portion ofthe conductive film 89 a is anisotropically etched using the resist mask91. After that, the resist mask 91 is removed.

As a result, the source and drain electrodes 92 a having a smaller areathan the conductive film 89 a are formed. Then, the resist mask 91 isremoved. FIG. 7B corresponds to a cross-sectional view taken along theline A-B of FIG. 9B. As illustrated in FIG. 9B, it can be seen that theend portions of the source and drain regions 89 are located outside theend portions of the source and drain electrodes 92 a. In addition, theend portions of the buffer layer 88 are located outside the source anddrain electrodes 92 a and the source and drain regions 89. The sourceand drain electrodes 92 a are isolated from each other and not connectedto an electrode formed in an adjacent pixel. In this case, by using theresist mask 91 formed by ashing of the resist mask 86, the source anddrain electrodes 92 a are formed. Alternatively, as illustrated in FIGS.1A and 1B, FIGS. 2A to 2C, FIGS. 3A and 3B, and FIGS. 4A and 4B, wetetching using the resist mask 86 may be conducted to form the source anddrain electrodes 92 a to 92 c.

As illustrated in FIG. 7B, with a shape in which the end portions of thesource and drain electrode 92 a and the end portions of the source anddrain region 89 are not aligned, the end portions of the source anddrain electrodes 92 a are apart from each other; therefore, leakagecurrent and short circuit between the source and drain electrodes can beprevented. Accordingly, a thin film transistor with high reliability andhigh withstand voltage can be manufactured.

Next, as illustrated in FIG. 7C, an insulating film 76 is formed overthe source and drain electrodes 92 a, the source and drain regions 89,the buffer layer 88, and the gate insulating film 52 b. The insulatingfilm 76 can be formed in a similar manner to the gate insulating films52 a and 52 b.

Next, as illustrated in FIG. 8A, a contact hole is formed in theinsulating film 76; stacked wirings 93 b and 93 c are formed in thecontact hole to be in contact with one of the source and drainelectrodes 92 a. Note that FIG. 8A corresponds to a cross-sectional viewtaken along the line A-B of FIG. 9C. The wirings 93 b and 93 c arewirings to be connected to a source or drain electrode formed in anadjacent pixel.

Next, a contact hole is formed in the insulating film 76, and a pixelelectrode 77 is formed in the contact hole to be in contact with theother of the source and drain electrodes 92 a, as illustrated in FIG.8B. Note that FIG. 8B corresponds to a cross-sectional view taken alonga line A-B of FIG. 9D.

Through the above-described process, a channel-etch thin film transistor84 can be formed. This channel-etch thin film transistor requires asmaller number of manufacturing steps and can achieve cost reduction. Byformation of a channel formation region with a microcrystallinesemiconductor film, a field-effect mobility of 1 cm²/V·sec to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scan line (gate line) side driver circuit.

In accordance with this embodiment mode, a highly reliable thin filmtransistor with excellent electric characteristics can be manufactured.

Embodiment Mode 2

In this embodiment mode, a liquid crystal display device including athin film transistor shown in Embodiment Mode 1 is described below.

First, a vertical alignment (VA) liquid crystal display device isdescribed. The VA liquid crystal display device is a kind of mode inwhich alignment of liquid crystal molecules of a liquid crystal panel iscontrolled. The VA liquid crystal display device is a mode in whichliquid crystal molecules are vertical to a panel surface when voltage isnot applied. In particular, in this embodiment mode, it is devised thata pixel is divided into several regions (sub-pixels) so that moleculesare aligned in different directions in the respective regions. This isreferred to as domain multiplication or multi-domain design. In thefollowing description, a liquid crystal display device with multi-domaindesign is described.

FIGS. 16 and 17 show a pixel electrode and a counter electrode,respectively. FIG. 16 is a plan view of a side of a substrate on whichthe pixel electrode is formed. FIG. 15 shows a cross-sectional structuretaken along the line A-B in FIG. 16. FIG. 17 is a plan view of a side ofa substrate on which the counter electrode is formed. Hereinafter,description is made with reference to these drawings.

FIG. 15 illustrates a state in which a substrate 600 provided with a TFT628, a pixel electrode 624 connected to the TFT 628, and a storagecapacitor portion 630 overlaps with a counter substrate 601 providedwith a counter electrode 640 and the like, and a liquid crystal isinjected therebetween.

At the position where the counter substrate 601 is provided with aspacer 642, a light shielding film 632, a first color film 634, a secondcolor film 636, a third color film 638, and the counter electrode 640are formed. With this structure, the height of a projection 644 forcontrolling alignment of the liquid crystal and the height of the spacer642 vary. An alignment film 648 is formed over the pixel electrode 624.Similarly, the counter electrode 640 is provided with an alignment film646. A liquid crystal layer 650 is formed between the alignment films646 and 648.

Although a columnar spacer is used for the spacer 642 in this embodimentmode, a bead spacer may be dispersed. Further, the spacer 642 may beformed over the pixel electrode 624 provided over the substrate 600.

The TFT 628, the pixel electrode 624 connected to the TFT 628, and thestorage capacitor portion 630 are formed over the substrate 600. Thepixel electrode 624 is connected to a wiring 618 via a contact hole 623which penetrates an insulating film 620 which covers the TFT 628, thewiring, and the storage capacitor portion 630 and also penetrates aninsulating film 622 which covers the insulating film 620. The thin filmtransistor shown in Embodiment Mode 1 can be used as the TFT 628 asappropriate. The storage capacitor portion 630 includes a firstcapacitor wiring 604 which is formed in a similar manner to a gatewiring 602 of the TFT 628, a gate insulating film 606, and a secondcapacitor wiring 617 which is formed in a similar manner to a wiring 616and the wiring 618.

A liquid crystal element is formed by overlapping of the pixel electrode624, the liquid crystal layer 650, and the counter electrode 640.

FIG. 16 shows a structure of the substrate 600 side. The pixel electrode624 is formed using a material described in Embodiment Mode 1. The pixelelectrode 624 is provided with a slit 625. The slit 625 is forcontrolling alignment of the liquid crystal.

A TFT 629, a pixel electrode 626 connected to the TFT 629, and a storagecapacitor portion 631 shown in FIG. 16 can be formed in a similar mannerto the TFT 628, the pixel electrode 624 and the storage capacitorportion 630, respectively. Both the TFT 628 and the TFT 629 areconnected to the wiring 616. A pixel of this liquid crystal panelincludes the pixel electrodes 624 and 626. Each of the pixel electrodes624 and 626 is a sub-pixel.

FIG. 17 shows a structure of the counter substrate side. The counterelectrode 640 is formed over the light shielding film 632. The counterelectrode 640 is preferably formed using a material similar to the pixelelectrode 624. The projection 644 for controlling alignment of theliquid crystal is formed over the counter electrode 640. Moreover, thespacer 642 is formed corresponding to the position of the lightshielding film 632.

FIG. 18 shows an equivalent circuit of this pixel structure. Both theTFT 628 and the TFT 629 are connected to the gate wiring 602 and thewiring 616. In this case, when potentials of the capacitor wiring 604and a capacitor wiring 605 are different from each other, operations ofliquid crystal elements 651 and 652 can vary. That is, alignment of theliquid crystal is precisely controlled and a viewing angle is increasedby individual control of potentials of the capacitor wirings 604 and605.

When voltage is applied to the pixel electrode 624 provided with theslit 625, electric field distortion (an oblique electric field) isgenerated near the slit 625. The slit 625 and the projection 644 on thecounter substrate 601 side are alternately arranged to be engaged witheach other and thus, an oblique electric field is effectively generatedto control alignment of the liquid crystal, so that a direction ofalignment of the liquid crystal varies depending on location. That is, aviewing angle of the liquid crystal panel is increased by domainmultiplication.

Next, another VA liquid crystal display device, which is different fromthe above-described device, is described with reference to FIGS. 19 to22.

FIGS. 19 and 20 each show a pixel structure of the VA liquid crystaldisplay device. FIG. 20 is a plan view of the substrate 600. FIG. 19shows a cross-sectional structure along a line Y-Z in FIG. 20.Hereinafter, description is made with reference to these drawings.

In this pixel structure, a plurality of pixel electrodes is included inone pixel, and a TFT is connected to each pixel electrode. Each TFT isdriven by a different gate signal. That is, a multi-domain pixel has astructure in which signals supplied to the respective pixel electrodesare individually controlled.

Via the contact hole 623, the pixel electrode 624 is connected to theTFT 628 using the wiring 618. Via a contact hole 627, the pixelelectrode 626 is connected to the TFT 629 using a wiring 619. The gatewiring 602 of the TFT 628 and a gate wiring 603 of the TFT 629 areseparated so that different gate signals can be given. In contrast, thewiring 616 functioning as a data line is used in common for the TFTs 628and 629. A first capacitor is formed by a capacitor wiring 690, the gateinsulating film 606 and the wiring 618. A second capacitor is formed bythe capacitor wiring 690, the gate insulating film 606 and the wiring619. As each of the TFTs 628 and 629, the thin film transistor shown inEmbodiment Mode 1 can be used as appropriate.

The pixel electrodes 624 and 626 have different shapes and are separatedby the slit 625. The pixel electrode 626 is formed so as to surround theexternal side of the pixel electrode 624 which is spread in a V-shape.Timing of voltage application is made to vary between the pixelelectrodes 624 and 626 by the TFTs 628 and 629 in order to controlalignment of the liquid crystal. FIG. 22 shows an equivalent circuit ofthis pixel structure. The TFT 628 is connected to the gate wiring 602.The TFT 629 is connected to the gate wiring 603. When different gatesignals are supplied to the gate wirings 602 and 603, operation timingsof the TFTs 628 and 629 can vary.

The counter substrate 601 is provided with the light shielding film 632,the second color film 636, and the counter electrode 640. Moreover, aplanarization film 637 is formed between the second color film 636 andthe counter electrode 640 to prevent alignment disorder of the liquidcrystal. FIG. 21 shows a structure of the counter substrate side. A slit641 is formed in the counter electrode 640, which is commonly used fordifferent pixels. The slit 641 and the slit 625 on the pixel electrodes624 and 626 side are alternately arranged to be engaged with each other;thus, an oblique electric field is effectively generated, and alignmentof the liquid crystal can be controlled. Accordingly, a direction inwhich the liquid crystal is aligned can vary depending on location, anda viewing angle is increased.

A first liquid crystal element is formed by overlapping of the pixelelectrode 624, the liquid crystal layer 650, and the counter electrode640. A second liquid crystal element is formed by overlapping of thepixel electrode 626, the liquid crystal layer 650, and the counterelectrode 640. This is a multi-domain structure in which the firstliquid crystal element and the second liquid crystal element areincluded in one pixel.

Next, a horizontal electric field liquid crystal display device isdescribed. The horizontal electric field mode is a method in which anelectric field is horizontally applied to liquid crystal molecules in acell, whereby the liquid crystal is driven to express a gray scale. Bythis method, a viewing angle can be increased to approximately 180degrees. Hereinafter, a liquid crystal display device employing thehorizontal electric field mode is described.

FIG. 23 illustrates a state in which the substrate 600 provided with theTFT 628 and the pixel electrode 624 connected to the TFT 628 overlapswith the counter substrate 601, and liquid crystal is injectedtherebetween. The counter substrate 601 is provided with the lightshielding film 632, the second color film 636, the planarization film637, and the like. Since the pixel electrode is provided on thesubstrate 600 side, it is not provided on the counter substrate 601side. The liquid crystal layer 650 is formed between the substrate 600and the counter substrate 601.

A first pixel electrode 607, the capacitor wiring 604 connected to thefirst pixel electrode 607, and the TFT 628 shown in Embodiment Mode 1are formed over the substrate 600. The first pixel electrode 607 can beformed using a material similar to the pixel electrode 77 shown inEmbodiment Mode 1. The first pixel electrode 607 is formed in a shapewhich is compartmentalized roughly in accordance with a pixel shape.Note that the gate insulating film 606 is formed over the first pixelelectrode 607 and the capacitor wiring 604.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 serves as a data line extending inone direction, through which a video signal is transmitted in a liquidcrystal panel, and is connected to a source region 610 and serves as oneof a source electrode and a drain electrode. The wiring 618 serves asthe other of the source electrode and the drain electrode, and isconnected to the second pixel electrode 624.

The insulating film 620 is formed over the wirings 616 and 618. Over theinsulating film 620, the second pixel electrode 624 connected to thewiring 618 via a contact hole 623 formed in the insulating film 620 isformed. The pixel electrode 624 is formed using a material similar tothe pixel electrode 77 shown in Embodiment Mode 1.

In such a manner, the TFT 628 and the second pixel electrode 624connected to the TFT 628 are formed over the substrate 600. Note that astorage capacitor is formed between the first pixel electrode 607 andthe second pixel electrode 624.

FIG. 24 is a plan view illustrating a structure of the pixel electrode.The pixel electrode 624 is provided with the slit 625. The slit 625 isfor controlling alignment of the liquid crystal. In this case, anelectric field is generated between the first pixel electrode 607 andthe second pixel electrode 624. The thickness of the gate insulatingfilm 606 formed between the first pixel electrode 607 and the secondpixel electrode 624 is 50 to 200 nm, which is sufficiently thin comparedwith the liquid crystal layer with a thickness of 2 to 10 μm.Accordingly, an electric field is generated substantially in parallel(in a horizontal direction) to the substrate 600. Alignment of theliquid crystal is controlled by the electric field. The liquid crystalmolecules are horizontally rotated using the electric field which isapproximately parallel to the substrate. In this case, since the liquidcrystal molecules are parallel to the substrate in any state, there arefew influences of contrast or the like depending on the angle ofviewing; thus, the viewing angle is expanded. Further, since both thefirst pixel electrode 607 and the second pixel electrode 624 arelight-transmitting electrodes, an aperture ratio can be increased.

Next, another example of a horizontal electric field liquid crystaldisplay device is described.

FIGS. 25 and 26 each show a pixel structure of an IPS liquid crystaldisplay device. FIG. 26 is a plan view. FIG. 25 shows a cross-sectionalstructure along a line A-B in FIG. 26. Hereinafter, description is madewith reference to these drawings.

FIG. 25 illustrates a state in which the substrate 600 provided with theTFT 628 and the pixel electrode 624 connected to the TFT 628 overlapswith the counter substrate 601, and liquid crystal is injectedtherebetween. The counter substrate 601 is provided with the lightshielding film 632, the second color film 636, the planarization film637, and the like. Since the pixel electrode is provided on thesubstrate 600 side, it is not provided on the counter substrate 601side. The liquid crystal layer 650 is formed between the substrate 600and the counter substrate 601.

A common potential line 609 and the TFT 628 shown in Embodiment Mode 1are formed over the substrate 600. The common potential line 609 can beformed at the same time as the gate wiring 602 of the TFT 628.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 serves as a data line extending inone direction, through which a video signal is transmitted in a liquidcrystal panel, and is connected to the source region 610 and serves asone of a source electrode and a drain electrode. The wiring 618 servesas the other of the source electrode and the drain electrode, and isconnected to the second pixel electrode 624.

The second insulating film 620 is formed over the wirings 616 and 618.Over the insulating film 620, the pixel electrode 624 connected to thewiring 618 via the contact hole 623 formed in the insulating film 620 isformed. The pixel electrode 624 is formed using a material similar tothe pixel electrode 77 shown in Embodiment Mode 1. Note that as shown inFIG. 26, the pixel electrode 624 is formed so as to generate ahorizontal electric field with a comb-shaped electrode which is formedat the same time as the common potential line 609. Moreover, the pixelelectrode 624 is formed so that comb-teeth portions of the pixelelectrode 624 are alternately engaged with the comb-shaped electrodewhich is formed at the same time as the common potential line 609.

Alignment of the liquid crystal is controlled by an electric fieldgenerated between a potential applied to the pixel electrode 624 and apotential of the common potential line 609. The liquid crystal moleculesare horizontally rotated using the electric field which is approximatelyparallel to the substrate. In this case, since the liquid crystalmolecules are parallel to the substrate in any state, there are a fewinfluences of contrast or the like depending on the angle of viewing;thus, the viewing angle is expanded.

In such a manner, the TFT 628 and the pixel electrode 624 connected tothe TFT 628 are formed over the substrate 600. A storage capacitor isformed by the common potential line 609, a capacitor electrode 615, andthe gate insulating film 606 provided therebetween. The capacitorelectrode 615 and the pixel electrode 624 are connected via a contacthole 633.

Next, a mode of a TN liquid crystal display device is described.

FIGS. 27 and 28 each show a pixel structure of a TN liquid crystaldisplay device. FIG. 28 is a plan view. FIG. 27 shows a cross-sectionalstructure taken along the line A-B in FIG. 28. Hereinafter, descriptionis made with reference to these drawings.

The pixel electrode 624 is connected to the TFT 628 by the wiring 618via the contact hole 623. The wiring 616 functioning as a data line isconnected to the TFT 628. As the TFT 628, any of the IP is shown inEmbodiment Mode 1 can be used.

The pixel electrode 624 is formed using the pixel electrode 77 shown inEmbodiment Mode 1.

The counter substrate 601 is provided with the light shielding film 632,the second color film 636, and the counter electrode 640. Theplanarization film 637 is formed between the second color film 636 andthe counter electrode 640 to prevent alignment disorder of liquidcrystal. The liquid crystal layer 650 is formed between the pixelelectrode 624 and the counter electrode 640.

A liquid crystal element is formed by overlapping of the pixel electrode624, the liquid crystal layer 650, and the counter electrode 640.

The counter electrode 640 can be formed using a material similar to thepixel electrode 624. The liquid crystal element is formed by overlappingof the pixel electrode 624, the liquid crystal layer 650, and thecounter electrode 640.

In any of the liquid crystal display devices shown in FIGS. 15 to 28,the substrate 600 or the counter substrate 601 may be provided with acolor filter, a shielding film (a black matrix) for preventingdisclination, or the like. Further, a polarizing plate is attached to asurface of the substrate 600, which is opposite to a surface of thesubstrate 600 on which the thin film transistor is formed. Moreover, apolarizing plate is attached to a surface of the counter substrate 601,which is opposite to a surface of the substrate on which the counterelectrode 640 is formed.

Through the above-described steps, the liquid crystal display device canbe formed. Since a thin film transistor with small off current andhighly-reliable electric properties is used in the liquid crystaldisplay device of this embodiment mode, the liquid crystal displaydevice has high contrast and high visibility. Further, since a thin filmtransistor in which a microcrystalline silicon film which is formedwithout a laser crystallization process is used for a channel formationregion is used, a liquid crystal display device with high visibility canbe formed with high productivity.

Embodiment Mode 3

Next, a structure of a display panel, which is one mode of a liquidcrystal display device of the present invention, is described below.

FIG. 12A shows a mode of a display panel in which a signal line drivercircuit 6013 which is separately formed is connected to a pixel portion6012 formed over a substrate 6011. The pixel portion 6012 and a scanline driver circuit 6014 are each formed using a thin film transistor inwhich a microcrystalline semiconductor film is used for a channelformation region. When the signal line driver circuit is formed using atransistor which has higher field-effect mobility compared with the thinfilm transistor in which the microcrystalline semiconductor film is usedfor the channel formation region, an operation of the signal line drivercircuit which demands higher driving frequency than that of the scanline driver circuit can be stabilized. Note that the signal line drivercircuit 6013 may be formed using a transistor in which a singlecrystalline semiconductor is used for a channel formation region, a thinfilm transistor in which a polycrystalline semiconductor is used for achannel formation region, or a transistor using SOI. The pixel portion6012, the signal line driver circuit 6013, and the scan line drivercircuit 6014 are each supplied with a potential of a power supply, avariety of signals, and the like via an FPC 6015.

Note that both the signal line driver circuit and the scan line drivercircuit may be formed over the same substrate as the pixel portion.

When a driver circuit is separately formed, a substrate over which thedriver circuit is formed is not necessarily attached to a substrate overwhich a pixel portion is formed, and may be attached over an FPC, forexample. FIG. 12B shows a mode of a display panel in which a signal linedriver circuit 6023 is separately formed and is connected to a pixelportion 6022 formed over a substrate 6021. The pixel portion 6022 andthe scan line driver circuit 6024 are each formed using a thin filmtransistor in which a microcrystalline semiconductor film is used for achannel formation region. The signal line driver circuit 6023 isconnected to the pixel portion 6022 via an FPC 6025. The pixel portion6022, the signal line driver circuit 6023, and the scan line drivercircuit 6024 are each supplied with a potential of a power supply, avariety of signals, and the like via the FPC 6025.

Alternatively, only part of a signal line driver circuit or part of ascan line driver circuit may be formed over the same substrate as apixel portion by using a thin film transistor in which amicrocrystalline semiconductor film is used for a channel formationregion, and the other part of the driver circuit may be separatelyformed and electrically connected to the pixel portion. FIG. 12C shows amode of a display panel in which an analog switch 6033 a included in asignal line driver circuit is formed over a substrate 6031, which is thesame substrate as a pixel portion 6032 and a scan line driver circuit6034, and a shift register 6033 b included in the signal line drivercircuit is separately formed over a different substrate and attached tothe substrate 6031. The pixel portion 6032 and the scan line drivercircuit 6034 are each formed using a thin film transistor in which amicrocrystalline semiconductor film is used for a channel formationregion. The shift register 6033 b included in the signal line drivercircuit is connected to the pixel portion 6032 via an FPC 6035. Thepixel portion 6032, the signal line driver circuit, and the scan linedriver circuit 6034 are each supplied with a potential of a powersupply, a variety of signals, and the like via the FPC 6035.

As shown in FIGS. 12A to 12C, in liquid crystal display devices of thepresent invention, all or a part of the driver circuit can be formedover the same substrate as the pixel portion, using the thin filmtransistor in which the microcrystalline semiconductor film is used forthe channel formation region.

Note that a connection method of a substrate which is separately formedis not particularly limited, and a known COG method, wire bondingmethod, TAB method, or the like can be used. Further, a connectionposition is not limited to the positions shown in FIGS. 12A to 12C aslong as electrical connection is possible. Moreover, a controller, aCPU, a memory, or the like may be formed separately and connected.

Note that a signal line driver circuit used in the present invention isnot limited to a mode including only a shift register and an analogswitch. In addition to the shift register and the analog switch, anothercircuit such as a buffer, a level shifter, or a source follower may beincluded. Moreover, the shift register and the analog switch are notnecessarily provided. For example, a different circuit such as a decodercircuit by which a signal line can be selected may be used instead ofthe shift register, or a latch or the like may be used instead of theanalog switch.

FIG. 30 is a block diagram of a liquid crystal display device of thepresent invention. The liquid crystal display device shown in FIG. 30includes a pixel portion 700 including a plurality of pixels eachprovided with a liquid crystal element, a scan line driver circuit 702which selects each pixel, and a signal line driver circuit 703 whichcontrols input of a video signal to a selected pixel.

In FIG. 30, the signal line driver circuit 703 includes a shift register704 and an analog switch 705. A clock signal (CLK) and a start pulsesignal (SP) are input to the shift register 704. When the clock signal(CLK) and the start pulse signal (SP) are input, a timing signal isgenerated in the shift register 704 and input to the analog switch 705.

A video signal is supplied to the analog switch 705. The analog switch705 samples the video signal in accordance with the timing signal andsupplies the resulting signal to a signal line of the next stage.

Next, a structure of the scan line driver circuit 702 is described. Thescan line driver circuit 702 includes a shift register 706 and a buffer707. The scan line driver circuit 702 may also include a level shifterin some cases. In the scan line driver circuit 702, when the clocksignal (CLK) and the start pulse signal (SP) are input to the shiftregister 706, a selection signal is generated. The generated selectionsignal is buffered and amplified by the buffer 707, and the resultingsignal is supplied to a corresponding scan line. Gates of transistors inpixels of one line are connected to the scan line. Further, since thetransistors in the pixels of one line should be turned on at the sametime, a buffer through which large current can flow is used as thebuffer 707.

In a full color liquid crystal display device, when video signalscorresponding to R (red), G (green), or B (blue) are sequentiallysampled and supplied to a corresponding signal line, the number ofterminals for connecting the shift register 704 and the analog switch705 corresponds to approximately ⅓ of the number of terminals forconnecting the analog switch 705 and the signal line in the pixelportion 700. Accordingly, when the analog switch 705 and the pixelportion 700 are formed over the same substrate, the number of terminalsused for connecting substrates which are separately formed can besuppressed compared with the case where the analog switch 705 and thepixel portion 700 are formed over different substrates; thus, occurrenceprobability of defective connection can be suppressed, and yield can beincreased.

Note that although the scan line driver circuit 702 shown in FIG. 30includes the shift register 706 and the buffer 707, the scan line drivercircuit 702 may include the shift register 706.

Note that structures of the signal line driver circuit and the scan linedriver circuit are not limited to the structures shown in FIG. 30, whichare merely one mode of the liquid crystal display device of the presentinvention.

Next, one mode of a shift register including thin film transistors inwhich microcrystalline semiconductor films with the same polarity areused for channel formation regions is described with reference to FIGS.31 and 32. FIG. 31 illustrates a structure of a shift register in thisembodiment mode. The shift register shown in FIG. 31 includes aplurality of flip-flops (flip-flops 701-1 to 701-n). The shift registeris operated by input of a first clock signal, a second clock signal, astart pulse signal, and a reset signal.

Connection relations of the shift register in FIG. 31 are described. Inthe i-th stage flip-flop 701-i (one of the flip-flops 701-1 to 701-n) inthe shift register of FIG. 31, a first wiring 501 shown in FIG. 32 isconnected to a seventh wiring 717-(i−1); a second wiring 502 shown inFIG. 32 is connected to a seventh wiring 717-(i+1); a third wiring 503shown in FIG. 32 is connected to a seventh wiring 717-i; and a sixthwiring 506 shown in FIG. 32 is connected to a fifth wiring 715.

Further, a fourth wiring 504 shown in FIG. 32 is connected to a secondwiring 712 in flip-flops of odd-numbered stages, and is connected to athird wiring 713 in flip-flops of even-numbered stages. A fifth wiring505 shown in FIG. 32 is connected to a fourth wiring 714.

Note that the first wiring 501 of the first stage flip-flop 701-1 shownin FIG. 32 is connected to a first wiring 711. Moreover, the secondwiring 502 of the n-th stage flip-flop 701-n shown in FIG. 32 isconnected to a sixth wiring 716.

Note that the first wiring 711, the second wiring 712, the third wiring713, and the sixth wiring 716 may be referred to as a first signal line,a second signal line, a third signal line, and a fourth signal line,respectively. The fourth wiring 714 and the fifth wiring 715 may bereferred to as a first power supply line and a second power supply line,respectively.

Next, FIG. 32 shows details of the flip-flop shown in FIG. 31. Aflip-flop shown in FIG. 32 includes a first thin film transistor 171, asecond thin film transistor 172, a third thin film transistor 173, afourth thin film transistor 174, a fifth thin film transistor 175, asixth thin film transistor 176, a seventh thin film transistor 177, andan eighth thin film transistor 178. In this embodiment mode, each of thefirst thin film transistor 171, the second thin film transistor 172, thethird thin film transistor 173, the fourth thin film transistor 174, thefifth thin film transistor 175, the sixth thin film transistor 176, theseventh thin film transistor 177, and the eighth thin film transistor178 is an n-channel transistor and is turned on when the gate-sourcevoltage (V_(gs)) exceeds the threshold voltage (V_(th)).

Next, connection structures of the flip-flop shown in FIG. 32 aredescribed below.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 171 is connected to the fourth wiring504. A second electrode (the other of the source electrode and the drainelectrode) of the first thin film transistor 171 is connected to thethird wiring 503.

A first electrode of the second thin film transistor 172 is connected tothe sixth wiring 506. A second electrode of the second thin filmtransistor 172 is connected to the third wiring 503.

A first electrode of the third thin film transistor 173 is connected tothe fifth wiring 505. A second electrode of the third thin filmtransistor 173 is connected to a gate electrode of the second thin filmtransistor 172. A gate electrode of the third thin film transistor 173is connected to the fifth wiring 505.

A first electrode of the fourth thin film transistor 174 is connected tothe sixth wiring 506. A second electrode of the fourth thin filmtransistor 174 is connected to the gate electrode of the second thinfilm transistor 172. A gate electrode of the fourth thin film transistor174 is connected to a gate electrode of the first thin film transistor171.

A first electrode of the fifth thin film transistor 175 is connected tothe fifth wiring 505. A second electrode of the fifth thin filmtransistor 175 is connected to the gate electrode of the first thin filmtransistor 171. A gate electrode of the fifth thin film transistor 175is connected to the first wiring 501.

A first electrode of the sixth thin film transistor 176 is connected tothe sixth wiring 506. A second electrode of the sixth thin filmtransistor 176 is connected to the gate electrode of the first thin filmtransistor 171. A gate electrode of the sixth thin film transistor 176is connected to the gate electrode of the second thin film transistor172.

A first electrode of the seventh thin film transistor 177 is connectedto the sixth wiring 506. A second electrode of the seventh thin filmtransistor 177 is connected to the gate electrode of the first thin filmtransistor 171. A gate electrode of the seventh thin film transistor 177is connected to the second wiring 502. A first electrode of the eighththin film transistor 178 is connected to the sixth wiring 506. A secondelectrode of the eighth thin film transistor 178 is connected to thegate electrode of the second thin film transistor 172. A gate electrodeof the eighth thin film transistor 178 is connected to the first wiring501.

Note that the point at which the gate electrode of the first thin filmtransistor 171, the gate electrode of the fourth thin film transistor174, the second electrode of the fifth thin film transistor 175, thesecond electrode of the sixth thin film transistor 176, and the secondelectrode of the seventh thin film transistor 177 are connected isreferred to as a node 143. The point at which the gate electrode of thesecond thin film transistor 172, the second electrode of the third thinfilm transistor 173, the second electrode of the fourth thin filmtransistor 174, the gate electrode of the sixth thin film transistor176, and the second electrode of the eighth thin film transistor 178 areconnected is referred to as a node 144.

Note that the first wiring 501, the second wiring 502, the third wiring503, and the fourth wiring 504 may be referred to as a first signalline, a second signal line, a third signal line, and a fourth signalline, respectively. The fifth wiring 505 and the sixth wiring 506 may bereferred to as a first power supply line and a second power supply line,respectively.

FIG. 33 shows an example of a top plan view of the flip-flop shown inFIG. 32.

A conductive film 901 includes a portion functioning as the firstelectrode of the first thin film transistor 171, and is connected to thefourth wiring 504 through a wiring 951 which is formed at the same timeas a pixel electrode.

A conductive film 902 includes a portion functioning as the secondelectrode of the first thin film transistor 171, and is connected to thethird wiring 503 through a wiring 952 which is formed at the same timeas the pixel electrode.

A conductive film 903 includes portions functioning as the gateelectrode of the first thin film transistor 171 and the gate electrodeof the fourth thin film transistor 174.

A conductive film 904 includes portions functioning as the firstelectrode of the second thin film transistor 172, the first electrode ofthe sixth thin film transistor 176, the first electrode of the fourththin film transistor 174, and the first electrode of the eighth thinfilm transistor 178, and the conductive film 904 is connected to thesixth wiring 506.

A conductive film 905 includes a portion functioning as the secondelectrode of the second thin film transistor 172, and is connected tothe third wiring 503 through a wiring 954 which is formed at the sametime as the pixel electrode.

A conductive film 906 includes portions functioning as the gateelectrode of the second thin film transistor 172 and the gate electrodeof the sixth thin film transistor 176.

A conductive film 907 includes a portion functioning as the firstelectrode of the third thin film transistor 173, and is connected to thefifth wiring 505 through a wiring 955.

A conductive film 908 includes portions functioning as the secondelectrode of the third thin film transistor 173 and the second electrodeof the fourth thin film transistor 174, and is connected to theconductive film 906 through a wiring 956 which is formed at the sametime as the pixel electrode.

A conductive film 909 includes a portion functioning as the gateelectrode of the third thin film transistor 173, and is connected to thefifth wiring 505 through the wiring 955.

A conductive film 910 includes a portion functioning as the firstelectrode of the fifth thin film transistor 175, and is connected to thefifth wiring 505 through a wiring 959 which is formed at the same timeas the pixel electrode.

A conductive film 911 includes portions functioning as the secondelectrode of the fifth thin film transistor 175 and the second electrodeof the seventh thin film transistor 177, and is connected to theconductive film 903 through a wiring 958 which is formed at the sametime as the pixel electrode.

A conductive film 912 includes a portion functioning as the gateelectrode of the fifth thin film transistor 175, and is connected to thefirst wiring 501 through a wiring 960 which is formed at the same timeas the pixel electrode.

A conductive film 913 includes a portion functioning as the secondelectrode of the sixth thin film transistor 176, and is connected to theconductive film 903 through a wiring 957 which is formed at the sametime as the pixel electrode.

A conductive film 914 includes a portion functioning as the gateelectrode of the seventh thin film transistor 177, and is connected tothe second wiring 502 through a wiring 962 which is formed at the sametime as the pixel electrode.

A conductive film 915 includes a portion functioning as the gateelectrode of the eighth thin film transistor 178, and is connected tothe conductive film 912 through a wiring 961 which is formed at the sametime as the pixel electrode.

A conductive film 916 includes a portion functioning as the secondelectrode of the eighth thin film transistor 178, and is connected tothe conductive film 906 through a wiring 953 which is formed at the sametime as the pixel electrode.

Note that parts of microcrystalline semiconductor films 981 to 988function as channel formation regions of the first to eighth thin filmtransistors, respectively.

When the circuits as shown in FIGS. 30 to 32 include a thin filmtransistor in which a microcrystalline semiconductor is used for achannel formation region, the circuits can be operated at high speed.For example, in comparison of a thin film transistor in which amicrocrystalline semiconductor film is used for a channel formationregion with a thin film transistor in which an amorphous semiconductorfilm is used for a channel formation region, the field-effect mobilityof the thin film transistor in which a microcrystalline semiconductorfilm is used for a channel formation region is higher, thus, drivingfrequency of a driver circuit (e.g., the shift register 706 in the scanline driver circuit 702) can be increased. Since the scan line drivercircuit 702 can be operated at high speed, frame frequency can beincreased or black frame insertion can be realized, for example.

When the frame frequency is increased, data for a screen is preferablygenerated in accordance with a direction of movement of an image. Thatis, motion compensation is preferably performed to interpolate data.When the frame frequency is increased and image data is interpolated insuch a manner, display characteristics of moving images are improved,and smooth display can be performed. For example, when frame frequencyis twice (e.g., 120 Hz or 100 Hz) or more, and preferably four times(e.g., 480 Hz or 400 Hz) or more, blurring and afterimages of movingimages can be reduced. In this case, the driving frequency of the scanline driver circuit 702 is also increased to be operated; thus, theframe frequency can be increased.

When black frame insertion is performed, image data or data for blackdisplay is supplied to the pixel portion 700. Thus, such a display modeis closer to impulse driving, and afterimages can be reduced. In thiscase, the driving frequency of the scan line driver circuit 702 is alsoincreased to be operated, and thus, black frame insertion can beperformed.

In addition, when the channel width of the thin film transistor in thescan line driver circuit 702 is increased or a plurality of scan linedriver circuits are provided, for example, higher frame frequency can berealized. For example, frame frequency can be eight times (e.g., 960 Hzor 800 Hz) or more. When a plurality of scan line driver circuits isprovided, a scan line driver circuit for driving even-numbered scanlines is provided on one side and a scan line driver circuit for drivingodd-numbered scan lines is provided on the opposite side; thus, increasein frame frequency can be realized. As an example, the channel width ofthe second thin film transistor 172 is preferably 300 μm or more, andmore preferably 1000 μm or more.

When the circuits as shown in FIGS. 30 to 32 include a thin filmtransistor in which a microcrystalline semiconductor is used for achannel formation region, the layout area can be reduced. Accordingly, aframe of the liquid crystal display device can be reduced. For example,in comparison of a thin film transistor in which a microcrystallinesemiconductor film is used for a channel formation region with a thinfilm transistor in which an amorphous semiconductor film is used for achannel formation region, the field-effect mobility of the thin filmtransistor in which a microcrystalline semiconductor film is used for achannel formation region is higher; thus, the channel width of the thinfilm transistor can be reduced. As a result, a frame of the liquidcrystal display device can be narrowed. As an example, the channel widthof the second thin film transistor 172 is preferably 3000 μm or less,and more preferably 2000 μm or less.

Note that in the second thin film transistor 172 of FIG. 32, a periodduring which a low-level signal is output to the third wiring 503 islong. In this period, the second thin film transistor 172 is kept on.Therefore, extreme stress is applied to the second thin film transistor172, and characteristics of the transistor are likely to deteriorate.When the characteristics of the transistor deteriorate, the thresholdvoltage is gradually increased. Thus, a current value is decreased. Inorder to supply enough current even when the transistor deteriorates,the channel width of the second thin film transistor 172 is preferablylarge. Alternatively, compensation is preferably done so that a circuitoperation is not affected even when the transistor deteriorates. Forexample, it is preferable that a transistor be provided in parallel withthe second thin film transistor 172, and the transistor and the secondthin film transistor 172 be made to be alternately turned on, so thatinfluence by deterioration is small.

In comparison of a thin film transistor in which a microcrystallinesemiconductor film is used for a channel formation region with a thinfilm transistor in which an amorphous semiconductor film is used for achannel formation region, the thin film transistor in which amicrocrystalline semiconductor film is used for a channel formationregion is harder to deteriorate. Accordingly, when the microcrystallinesemiconductor film is used for the channel formation region, the channelwidth of the thin film transistor can be reduced. Alternatively, thetransistor can be normally operated without any circuit for compensationfor deterioration. Accordingly, the layout area can be reduced.

Next, the appearance and a cross section of a liquid crystal displaypanel which is one mode of the liquid crystal display device of thepresent invention are described with reference to FIGS. 29A and 29B.FIG. 29A is a top plan view of a panel. In the panel, a thin filmtransistor 4010 including a microcrystalline semiconductor film and aliquid crystal element 4013 which are formed over a first substrate 4001are sealed between the first substrate 4001 and a second substrate 4006by a sealing material 4005. FIG. 29B is a cross-sectional view along aline A-A′ in FIG. 29A.

The sealing material 4005 is provided so as to surround a pixel portion4002 and a scan line driver circuit 4004 which are provided over thefirst substrate 4001. The second substrate 4006 is provided over thepixel portion 4002 and the scan line driver circuit 4004. Accordingly,the pixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with liquid crystal 4008 by the first substrate 4001, thesealing material 4005, and the second substrate 4006. Further, a signalline driver circuit 4003 formed using a polycrystalline semiconductorfilm over a different substrate is mounted on a region over the firstsubstrate 4001, which is different from the region surrounded by thesealing material 4005. Note that in this embodiment mode, an example isdescribed in which the signal line driver circuit including a thin filmtransistor in which a polycrystalline semiconductor film is used for achannel formation region is attached to the first substrate 4001;however, a signal line driver circuit may be formed using a transistorin which a single crystalline semiconductor is used for a channelformation region and attached to a substrate. FIGS. 29A and 29Billustrate a thin film transistor 4009 formed using a polycrystallinesemiconductor film, which is included in the signal line driver circuit4003.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. FIG. 29B illustrates the thin film transistor4010 included in the pixel portion 4002. The thin film transistor 4010corresponds to a thin film transistor in which a microcrystallinesemiconductor film is used for a channel formation region.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010 through a wiring4040. A counter electrode 4031 of the liquid crystal element 4013 isformed on the second substrate 4006. A portion where the pixel electrode4030, the counter electrode 4031, and the liquid crystal 4008 overlapwith each other corresponds to the liquid crystal element 4013.

Note that for each of the first substrate 4001 and the second substrate4006, glass, metal (typically, stainless steel), ceramic, or plastic canbe used. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Moreover, a sheet in which aluminum foil is interposedbetween PVF films or polyester films can also be used.

A spherical spacer 4035 is provided to control a distance (a cell gap)between the pixel electrode 4030 and the counter electrode 4031. Notethat a spacer obtained by selective etching of an insulating film may beused.

A variety of signals and a potential supplied to the separately formedsignal line driver circuit 4003, the pixel portion 4002, and the scanline driver circuit 4004 are supplied from an FPC 4018 through leadwirings 4014 and 4015.

In this embodiment mode, a connection terminal 4016 is formed using thesame conductive film as the pixel electrode 4030 included in the liquidcrystal element 4013. Further, the lead wirings 4014 and 4015 are formedusing the same conductive film as the wiring 4040.

The connection terminal 4016 is electrically connected to a terminalincluded in the FPC 4018 through an anisotropic conductive film 4019.

Note that although not shown, a liquid crystal display device shown inthis embodiment mode includes an alignment film and a polarizing plate,and may also include a color filter and/or a shielding film.

FIGS. 29A and 29B illustrate an example in which the signal line drivercircuit 4003 is separately formed and attached to the first substrate4001; however, this embodiment mode is not limited to this structure. Ascan line driver circuit may be separately formed and attached to asubstrate, or only part of a signal line driver circuit or part of ascan line driver circuit may be separately formed and attached to asubstrate.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes.

Embodiment Mode 4

The liquid crystal display device obtained according to the presentinvention can be used for an active matrix liquid crystal module. Thatis, the present invention can be implemented in any of electronicdevices having a display portion in which such an active matrix liquidcrystal module is incorporated.

Examples of such electronic devices include cameras such as a videocamera and a digital camera, a head-mounted display (a goggle-typedisplay), a car navigation system, a projector, a car stereo, a personalcomputer, and a portable information terminal (e.g., a mobile computer,a cellular phone, and an e-book reader). FIGS. 13A to 13C show examplesof such electronic devices.

FIG. 13A shows a television device. The television device can becompleted by incorporating a display module into a housing as shown inFIG. 13A. A display panel with an FPC attached is also referred to as adisplay module. A main screen 2003 is formed using the display module,and other accessories such as a speaker portion 2009 and an operationswitch are provided. Thus, the television device can be completed.

As shown in FIG. 13A, a display panel 2002 using a liquid crystalelement is incorporated into a housing 2001. The television device canreceive general TV broadcast by a receiver 2005, and can be connected toa wired or wireless communication network via a modem 2004 so thatone-way (from a sender to a receiver) or two-way (between a sender and areceiver or between receivers) information communication can beperformed. The television device can be operated by a switchincorporated into the housing or a separate remote control unit 2006.The remote control unit may include a display portion 2007 fordisplaying information to be output.

Further, the television device may include a sub screen 2008 formedusing a second display panel for displaying channels, sound volume, andthe like, in addition to the main screen 2003. In this structure, themain screen 2003 may be formed using a liquid crystal display panel withan excellent viewing angle, and the sub screen may be formed using aliquid crystal display panel in which display is performed with lowpower consumption. Alternatively, when reduction in power consumption isprioritized, a structure may be employed in which the main screen 2003is formed using a liquid crystal display panel, the sub screen is formedusing a liquid crystal display panel, and the sub screen can be turnedon and off.

FIG. 14 is a block diagram of a main structure of a television device. Adisplay panel 900 is provided with a pixel portion 921. A signal linedriver circuit 922 and a scan line driver circuit 923 may be mounted onthe display panel 900 by a COG method.

As for other external circuits, the television device includes a videosignal amplifier circuit 925 which amplifies a video signal amongsignals received by a tuner 924; a video signal processing circuit 926which converts a signal output from the video signal amplifier circuit925 into a color signal corresponding to each color of red, green, andblue; a control circuit 927 which converts the video signal into aninput specification of a driver IC; and the like. The control circuit927 outputs signals to each of the scan line side and the signal lineside. When digital driving is performed, a structure may be employed inwhich a signal dividing circuit 928 is provided on the signal line sideand an input digital signal is divided into m signals to be supplied.

Among the signals received by the tuner 924, an audio signal istransmitted to an audio signal amplifier circuit 929, and an outputthereof is supplied to a speaker 933 through an audio signal processingcircuit 930. A control circuit 931 receives control information onreceiving station (receiving frequency) and volume from an input portion932 and transmits a signal to the tuner 924 and the audio signalprocessing circuit 930.

It is needless to say that the present invention is not limited to atelevision device and can be applied to a variety of uses, such as amonitor of a personal computer, a large display medium such as aninformation display board at the train station, the airport, or thelike, or an advertisement display board on the street.

FIG. 13B shows an example of a cellular phone 2201. The cellular phone2201 includes a display portion 2202, an operation portion 2203, and thelike. When the liquid crystal display device described in theabove-described embodiment mode is used for the display portion 2202,mass productivity can be increased.

A portable computer shown in FIG. 13C includes a main body 2401, adisplay portion 2402, and the like. When the liquid crystal displaydevice described in the above-described embodiment mode is used for thedisplay portion 2402, mass productivity can be increased.

Embodiment 1

A microcrystalline silicon film was formed, and results of measuring thecrystallinity of the film by Raman spectroscopy are shown in FIGS. 34Aand 34B.

The microcrystalline silicon film was formed under conditions where theRF power frequency was 13.56 MHz, the film formation temperature was280° C., the flow rate ratio of hydrogen to a silane gas was 100:1, andthe pressure was 280 Pa. FIG. 34A shows Raman scattering spectra andshows a comparison of measurement results of a microcrystalline siliconfilm that was formed with the amount of power of an RF power sourcebeing 100 W and a microcrystalline silicon film with 300 W.

Note that the crystalline peak position of single crystalline silicon isat 521 cm⁻¹. Note that, needless to say, the crystalline peak ofamorphous silicon cannot be observed, and only a broad peak is measuredat 480 cm⁻¹ as illustrated in FIG. 34B. The microcrystalline siliconfilm of this specification refers to one whose crystalline peak positioncan be observed at greater than or equal to 481 cm⁻¹ and less than 520cm⁻¹ when measured with a Raman spectrometer.

The crystalline peak position of the microcrystalline silicon film whichwas formed with the amount of power of an RF power source being 100 W isat 518.6 cm⁻¹; the full width at half maximum (FWHM) is 11.9 cm⁻¹; andthe crystalline/amorphous peak intensity ratio (I_(c)/I_(a)) is 4.1.

The crystalline peak position of the microcrystalline silicon film thatwas formed with the amount of power of an RF power source being 300 W isat 514.8 cm⁻¹; the full width at half maximum (FWHM) is 18.7 cm⁻¹; andthe crystalline/amorphous peak intensity ratio (I_(c)/I_(a)) is 4.4.

As illustrated in FIG. 34A, there is a crystal peak shift and a largedifference in full width at half maximum depending on RF power. It canbe considered that this is because the grain size is likely to be small,because ion bombardment becomes significant at high power and the growthof grains is inhibited. In addition, because the power frequency of aCVD apparatus, with which the microcrystalline silicon film used formeasurement of FIG. 34A was formed, is 13.56 MHz, thecrystalline/amorphous peak intensity ratio (I_(c)/I_(a)) is 4.1 or 4.4.However, it is also confirmed that the crystalline/amorphous peakintensity ratio (I_(c)/I_(a)) can be 6 if the RF power frequency is 27MHz. Accordingly, the crystalline/amorphous peak intensity ratio(I_(c)/I_(a)) can be further increased when the RF power frequency ishigher than 27 MHz, for example, when the RF power frequency is 2.45GHz.

Embodiment 2

In this embodiment, results of device simulation of transistorcharacteristics and electron density distribution of the thin filmtransistor of the present invention are described. For devicesimulation, the device simulator “ATLAS” made by Silvaco is used.

FIG. 35 shows a device structure. An insulating substrate 2301 isassumed to be a glass substrate (with a thickness of 0.5 μm) whichcontains silicon oxide (with a dielectric constant of 4.1) as its maincomponent. Note that, although the thickness of the insulating substrate2301 is often 0.5 mm, 0.7 mm, or the like in a practical manufacturingprocess, the thickness is defined to such a sufficient thickness that anelectric field at the lower surface of the insulating substrate 2301does not affect thin film transistor characteristics.

Over the insulating substrate 2301, a gate electrode 2303 formed ofmolybdenum (with a thickness of 150 nm) is stacked. The work function ofmolybdenum is 4.6 eV.

Over the gate electrode 2303, a gate insulating film 2305 having astacked structure of a silicon nitride film (with a dielectric constantof 7.0 and a thickness of 110 nm) and a silicon oxynitride film (with adielectric constant of 4.1 and a thickness of 110 nm) is stacked.

Over the gate insulating film 2305, a μc-Si film 2307 and an a-Si film2309 are stacked. Here, conditions are separately set for a stackedlayer of the μc-Si film 2307 with a thickness of 0 nm and the a-Si film2309 with a thickness of 100 nm, a stacked layer of the μc-Si film 2307with a thickness of 10 nm and the a-Si film 2309 with a thickness of 90nm, a stacked layer of the μc-Si film 2307 with a thickness of 50 nm andthe a-Si film 2309 with a thickness of 50 nm, a stacked layer of theμc-Si film 2307 with a thickness of 90 nm and the a-Si film 2309 with athickness of 10 nm, and a stacked layer of the μc-Si film 2307 with athickness of 100 am and the a-Si film 2309 with a thickness of 0 nm.

In addition, each of regions of the a-Si film 2309 overlapping with afirst a-Si(n⁺) film 2311 and a second a-Si(n⁺) film 2313 has anotherthickness of 50 nm in addition to the above-described thickness. Thatis, a part of the a-Si film 2309 in a region where the first a-Si(n⁺)film 2311 and the second a-Si(n⁺) film 2313 are not formed is etched by50 nm and has a concave portion.

Over the a-Si film 2309, the first a-Si(n⁺) film 2311 (with a thicknessof 50 nm) and the second a-Si(n⁺) film 2313 (with a thickness of 50 nm)are stacked. In the thin film transistor shown in FIG. 35, the distancebetween the first a-Si(n⁺) film 2311 and the second a-Si(n⁺) film 2313corresponds to a channel length L. Here, the channel length L is 6 μm.The channel width W is 15 μm.

Over the first a-Si(n⁺) film 2311 and the second a-Si(n⁺) film 2313, asource electrode 2315 and a drain electrode 2317, each of which isformed of molybdenum (Mo) (with a thickness of 300 nm), are formed,respectively. A contact between the source electrode 2315 and the firsta-Si(n⁺) film 2311 and that between the drain electrode 2317 and thesecond a-Si(n⁺) film 2313 are defined as ohmic contacts.

FIG. 36 shows results of DC characteristics (V_(g)−I_(d)characteristics, V_(d)=14 V) when device simulation of the thin filmtransistor shown in FIG. 35 is performed with the thicknesses of theμc-Si film and the a-Si film being changed variously. FIGS. 37A and 37Beach show electron concentration distribution in the thin filmtransistor when the μc-Si film 2307 has a thickness of 10 nm and thea-Si film has a thickness of 90 nm. FIG. 37A shows results of electronconcentration distribution when the thin film transistor is in an onstate (V_(g)=+10 V, V_(d)=14 V), and FIG. 37B shows results of electronconcentration distribution when the thin film transistor is in an offstate (V_(g)=−10 V, V_(d)=14 V).

It can be seen from FIG. 36 that, as the thickness of the a-Si film isincreased, the off current is decreased. In addition, when the thicknessof the a-Si film is 50 nm or more, the drain current when V_(g) is −20 Vcan be made to be lower than 1×10⁻¹³ A.

It can also be seen that, as the thickness of the μc-Si film isincreased, the on current is increased. In addition, when the thicknessof the μc-Si film is set to be 10 nm or more, the drain current whenV_(g) is 20 V can be made to be 1×10⁻⁵ A or more.

It can be seen from FIG. 37A that, in the on state, the μc-Si film has ahigher electron density than the a-Si film. That is, the μc-Si film withhigh electric conductivity has a high electron density; thus, it can beseen that, in the on state, electrons are easy to flow and the draincurrent is increased.

It can be seen from FIG. 37B that, in the off state, the a-Si film has ahigher electron density than the μc-Si film. That is, the a-Si film withlow electric conductivity has a high electron density; thus, it can beseen that, in the off state, electrons are difficult to flow and thedrain current is equivalent to that of a thin film transistor in whichan a-Si film is used as a channel formation region.

It can be seen from the above description that a thin film transistor asillustrated in FIG. 35, in which a μc-Si film is formed over a gateinsulating film, an a-Si film is formed over the μc-Si film, and sourceand drain regions are formed over the a-Si film, can be made to have alower off current and a higher on current.

This application is based on Japanese Patent Applications serial nos.2007-179092 filed with Japan Patent Office on Jul. 6, 2007, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A liquid crystal display device comprising: apixel comprising a first transistor, a second transistor, a first liquidcrystal element, a second liquid crystal element, a first capacitor, anda second capacitor, wherein one of a source electrode and a drainelectrode of the first transistor is electrically connected to a datawiring, wherein one of a source electrode and a drain electrode of thesecond transistor is electrically connected to the data wiring, whereinthe other of the source electrode and the drain electrode of the firsttransistor is electrically connected to a pixel electrode of the firstliquid crystal element and one electrode of the first capacitor, whereinthe other of the source electrode and the drain electrode of the secondtransistor is electrically connected to a pixel electrode of the secondliquid crystal element and one electrode of the second capacitor,wherein a gate electrode of the first transistor is electricallyconnected to a gate wiring, wherein a gate electrode of the secondtransistor is electrically connected to the gate wiring, wherein thefirst transistor and the second transistor each includes: a gateinsulating film over the gate electrode; a first semiconductor film overthe gate insulating film; a second semiconductor film over the firstsemiconductor film; and a third semiconductor film over the secondsemiconductor film, wherein a first conductive film, a second conductivefilm, and a third conductive film are positioned over the thirdsemiconductor film, wherein the first conductive film has a regionfunctioning as the one of the source electrode and the drain electrodeof the first transistor and a region functioning as the one of thesource electrode and the drain electrode of the second transistor,wherein the second conductive film has a region functioning as the otherof the source electrode and the drain electrode of the first transistorand a region functioning as the one electrode of the first capacitor,wherein the third conductive film has a region functioning as the otherof the source electrode and the drain electrode of the second transistorand a region functioning as the one electrode of the second capacitor,wherein an edge of the first conductive film has a region positionedinside edges of the first semiconductor film, the second semiconductorfilm, and the third semiconductor film included in the first transistor,wherein an edge of the second conductive film has a region positionedinside edges of the first semiconductor film, the second semiconductorfilm, and the third semiconductor film included in the first transistor,and wherein an edge of the third conductive film has a region positionedinside edges of the first semiconductor film, the second semiconductorfilm, and the third semiconductor film included in the secondtransistor.
 2. The liquid crystal display device according to claim 1,wherein the gate electrode of each of the first transistor and thesecond transistor includes titanium.
 3. The liquid crystal displaydevice according to claim 1, wherein the third semiconductor filmincludes an impurity element imparting one conductivity type.
 4. Theliquid crystal display device according to claim 1, wherein each of thefirst conductive film, the second conductive film, and the thirdconductive film includes copper.
 5. The liquid crystal display deviceaccording to claim 1, wherein the other electrode of the first capacitoris electrically connected to a first capacitor wiring, wherein the otherelectrode of the second capacitor is electrically connected to a secondcapacitor wiring, and wherein the gate wiring, the first capacitorwiring, and the second capacitor wiring extend in the same directionwhen seen from above.